Hook up Android closed captions 'enabled' setting to Blink

Source/core/html/HTMLMediaElement.cpp

 /*
  * Copyright (C) 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 2462 matching lines @@
 {
     if (!m_textTracks || !m_textTracks->length())
         return;
 
     AutomaticTrackSelection::Configuration configuration;
     if (m_processingPreferenceChange)
         configuration.disableCurrentlyEnabledTracks = true;
     if (m_closedCaptionsVisible)
         configuration.forceEnableSubtitleOrCaptionTrack = true;
 
+#if OS(ANDROID)
+    // Determine captions and subtitles visibility based on OS captions state only if the platform
+    // is Android because the setting is currently only available on Android.
+    Settings* settings = document().settings();
+    if (settings) {
+        if (!settings->textTracksEnabled())
+            return;
+        configuration.forceEnableSubtitleOrCaptionTrack = true;

[fs, 2015/05/05 15:09:15]

This flags forces _a_ track (any track) to be enabled. I.e. it disregards
language et.c. The provided "documentation" doesn't say anything about how these
cases should be handled (if for instance the specified language should be
preferred or required.)

It almost seems better to let this flag gate auto-selection entirely:

if (!settings || !settings->automaticTrackSelectionEnabled())
    return;

AutomaticTrackSelection trackSelection(...);
...

then defaulting it to 'true' would preserve the current behavior, and Android
could set it to 'false' by default if it wants to.

[philipj_slow, 2015/05/06 09:54:27]

This bit seems a bit odd to me as well. How about a setting which says which
kind of tracks to prefer in automatic text track selection? Its three states
would be "default" (only enable the default tracks), "subtitles" and "captions".
Whenever it changes, disable all tracks, reset any relevant state, and run
automatic text track selection again. Would that work?

[srivats, 2015/06/09 00:45:51]

Done.

+    }
+#endif
+
     AutomaticTrackSelection trackSelection(configuration);
     trackSelection.perform(*m_textTracks);
 
     textTracksChanged();
 }
 
 bool HTMLMediaElement::havePotentialSourceChild()
 {
     // Stash the current <source> node and next nodes so we can restore them after checking
     // to see there is another potential.
@@ skipping 1172 matching lines @@
 
 #if ENABLE(WEB_AUDIO)
 void HTMLMediaElement::clearWeakMembers(Visitor* visitor)
 {
     if (!visitor->isHeapObjectAlive(m_audioSourceNode) && audioSourceProvider())
         audioSourceProvider()->setClient(nullptr);
 }
 #endif
 
 }