[LoAF] Set sourceCharPosition for the invoke-type resolve-promise

The script entries of LoAF for the promise-resolve invoker type do not
provide sourceCharPosition. This CL enables the provision of the source
position.

- Introduced CapturePartialSourceLocationFromStack(v8::Isolate*) to
  retrieve both URL and starting character position of the currently
  executing script.
- Added LongAnimationFrameSourceCharPosition runtime flag.

Bug: 381529126
Test:
- external/wpt/long-animation-frame/tentative/loaf-source-
location.html
- external/wpt/long-animation-frame/tentative/loaf-source-location-redirect.html

Change-Id: I481a62d8eb4a5649fdddcff5b657d442335e8725
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6060398
Reviewed-by: Noam Rosenthal <[email protected]>
Commit-Queue: Joone Hur <[email protected]>
Reviewed-by: Nate Chapin <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1397921}

long-animation-frame/tentative/loaf-source-location-redirect.html

@@ -63,7 +63,7 @@ <h1>Long Animation Frame: source location with cross-origin redirects</h1>
         assert_not_equals(result, "empty", "CORS-ok scripts should expose sourceLocation");
       } else {
         assert_equals(result, "pre-redirect", "No-CORS classic scripts should expose pre-redirect URL");
-        assert_equals(script.sourceCharPosition, type === "script-block" ? 0 : -1, "No-CORS classic scripts should not expose character index");
+        assert_equals(script.sourceCharPosition, type === "script-block" || type === "resolve-promise" ? 0 : -1, "No-CORS classic scripts should not expose character index");
       }
     }, `Test ${type} with ${scriptType}`);
   }

long-animation-frame/tentative/loaf-source-location.html

@@ -51,7 +51,22 @@ <h1>Long Animation Frame: source location extraction</h1>
     document.body.appendChild(scriptElement);
   }, script => script.invoker === "Window.fetch.then", t);
   assert_true(script.sourceURL.includes("promise-generates-loaf.js"));
+  assert_equals(script.sourceCharPosition, 0);
 }, "Source location should be extracted for promises");
 
+// This test verifies that script entries of type "promise-resolve" correctly return
+// the sourceCharPosition value. It ensures that the reported position matches the
+// expected starting character position in the source code.
+promise_test(async t => {
+  const scriptLocation = new URL("resources/promise-generates-loaf-start-pos.js", location.href);
+  const [entry, script] = await expect_long_frame_with_script((t, busy_wait) => {
+    const scriptElement = document.createElement("script");
+    scriptElement.src = scriptLocation;
+    document.body.appendChild(scriptElement);
+  }, script => script.invoker === "Window.fetch.then", t);
+  assert_true(script.sourceURL.includes("promise-generates-loaf-start-pos.js"));
+  assert_equals(script.sourceCharPosition, 64);
+}, "SourceCharPosition should be 64 for promises");
+
 </script>
 </body>

long-animation-frame/tentative/resources/promise-generates-loaf-start-pos.js

@@ -0,0 +1,4 @@
+// The starting position of this code in the source file is 64.
+fetch("/common/dummy.xml").then(() => {
+  generate_loaf_now();
+});