Use GBK as fallback, not gb18030

It's equivalent for decoding, but gives more conservative encoding that's likely to be more compatible.

Fixes whatwg#4557.

source

@@ -106347,15 +106347,15 @@ dictionary <dfn dictionary>StorageEventInit</dfn> : <span>EventInit</span> {
     compatibility with legacy content. Locales are identified by BCP 47 language tags. <ref spec=BCP47> <ref spec=ENCODING></p>
 
     <!-- based on three sources:
-          1. mozilla 1.9.1 localizations: http://mxr.mozilla.org/l10n-mozilla1.9.1/find?string=global%2Fintl.properties&tree=l10n-mozilla1.9.1&hint= (though this data was apparently misinterpreted in some cases)
-          2. windows vista encodings: http://msdn.microsoft.com/en-us/goglobal/bb896001
-          3. chrome encodings: https://code.google.com/p/chromium/codesearch#search/&q=IDS_DEFAULT_ENCODING
+          1. Mozilla 1.9.1 localizations: http://mxr.mozilla.org/l10n-mozilla1.9.1/find?string=global%2Fintl.properties&tree=l10n-mozilla1.9.1&hint= (though this data was apparently misinterpreted in some cases)
+          2. Windows Vista encodings: http://msdn.microsoft.com/en-us/goglobal/bb896001
+          3. Chrome encodings: https://code.google.com/p/chromium/codesearch#search/&q=IDS_DEFAULT_ENCODING
          several assumptions were made in this process; amongst them:
           - ISO-8859-1 and windows-1252 are the same (supported by encoding.spec.whatwg.org)
           - ISO-8859-9 and windows-1254 are the same (supported by encoding.spec.whatwg.org)
           - windows-31J and Shift_JIS are the same (supported by encoding.spec.whatwg.org)
           - windows-932 is close enough to Shift_JIS to be treated as equivalent (supported by wikipedia)
-          - windows-936 is a basically a subset of GBK which is basically a subset of gb18030 (supported by wikipedia)
+          - windows-936 is basically the same as GBK which is a subset of gb18030 (supported by wikipedia), but use GBK for its conservative encoder
           - windows-950 is basically the same as Big5 (supported by wikipedia)
           - Firefox's UTF-8 defaults are all bogus
     -->
@@ -106722,17 +106722,17 @@ dictionary <dfn dictionary>StorageEventInit</dfn> : <span>EventInit</span> {
       <tr>
        <td>zh-CN
        <td>Chinese (People's Republic of China)
-       <td><span>gb18030</span> <!-- Windows Vista, Chrome, and Firefox agreed -->
+       <td><span>GBK</span> <!-- Windows Vista, Chrome, and Firefox agreed -->
 
       <!-- zh-HK, Chinese (Hong Kong S.A.R.), is not listed here because neither Chrome nor Firefox knew about it. For what it's worth, Windows Vista wanted Big5 -->
 
-      <!-- zh-Hans, Chinese (Simplified), is not listed here because neither Chrome nor Firefox knew about it. For what it's worth, Windows Vista wanted gb18030 -->
+      <!-- zh-Hans, Chinese (Simplified), is not listed here because neither Chrome nor Firefox knew about it. For what it's worth, Windows Vista wanted GBK -->
 
       <!-- zh-Hant, Chinese (Traditional), is not listed here because neither Chrome nor Firefox knew about it. For what it's worth, Windows Vista wanted Big5 -->
 
       <!-- zh-MO, Chinese (Macao S.A.R.), is not listed here because neither Chrome nor Firefox knew about it. For what it's worth, Windows Vista wanted Big5 -->
 
-      <!-- zh-SG, Chinese (Singapore), is not listed here because neither Chrome nor Firefox knew about it. For what it's worth, Windows Vista wanted gb18030 -->
+      <!-- zh-SG, Chinese (Singapore), is not listed here because neither Chrome nor Firefox knew about it. For what it's worth, Windows Vista wanted GBK -->
 
       <tr>
        <td>zh-TW
@@ -107138,7 +107138,7 @@ dictionary <dfn dictionary>StorageEventInit</dfn> : <span>EventInit</span> {
   <dfn data-x-href="https://encoding.spec.whatwg.org/#windows-1256">windows-1256</dfn>,
   <dfn data-x-href="https://encoding.spec.whatwg.org/#windows-1257">windows-1257</dfn>,
   <dfn data-x-href="https://encoding.spec.whatwg.org/#windows-1258">windows-1258</dfn>,
-  <dfn data-x-href="https://encoding.spec.whatwg.org/#gb18030">gb18030</dfn>,
+  <dfn data-x-href="https://encoding.spec.whatwg.org/#gbk">GBK</dfn>,
   <dfn data-x-href="https://encoding.spec.whatwg.org/#big5">Big5</dfn>,
   <dfn data-x-href="https://encoding.spec.whatwg.org/#iso-2022-jp">ISO-2022-JP</dfn>,
   <dfn data-x-href="https://encoding.spec.whatwg.org/#shift_jis">Shift_JIS</dfn>,