Hook up Android closed captions 'enabled' setting to Blink

Source/core/html/HTMLMediaElement.cpp

Index: Source/core/html/HTMLMediaElement.cpp
diff --git a/Source/core/html/HTMLMediaElement.cpp b/Source/core/html/HTMLMediaElement.cpp
index 5558692f2650d0baa1f5ee8941d302f798161f0d..5f93cdf67a0f97c31d932cd46edeb5d4c963d4fd 100644
--- a/Source/core/html/HTMLMediaElement.cpp
+++ b/Source/core/html/HTMLMediaElement.cpp
@@ -2480,6 +2480,17 @@ void HTMLMediaElement::honorUserPreferencesForAutomaticTextTrackSelection()
     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);