Emit disconnect_document with errorType to FRS on FF client disconnects

packages/drivers/driver-base/src/documentDeltaConnection.ts

@@ -419,7 +419,7 @@ export class DocumentDeltaConnection
 		);
 	}
 
-	protected disconnect(err: IAnyDriverError) {
+	protected disconnect = (err: IAnyDriverError) => {
 		// Can't check this.disposed here, as we get here on socket closure,
 		// so _disposed & socket.connected might be not in sync while processing
 		// "dispose" event.
@@ -455,7 +455,7 @@ export class DocumentDeltaConnection
 	 * Disconnect from the websocket.
 	 * @param reason - reason for disconnect
 	 */
-	protected disconnectCore(err: IAnyDriverError) {
+	protected disconnectCore = (err: IAnyDriverError) => {
 		this.socket.disconnect();
 	}