Expose APIs for text track settings

Source/core/frame/Settings.in

Index: Source/core/frame/Settings.in
diff --git a/Source/core/frame/Settings.in b/Source/core/frame/Settings.in
index 66d01deb40d0ec2b77b6c0f1ca5fb01d21574607..8752e7c7d5bad0ee1cf6faf19386389b6a3cdb48 100644
--- a/Source/core/frame/Settings.in
+++ b/Source/core/frame/Settings.in
@@ -325,3 +325,14 @@ logDnsPrefetchAndPreconnect initial=false
 # If true, the UA styles will include viewportAndroid.css. It is used for
 # Android, and dynamically set by the inspector for mobile emulation.
 useMobileViewportStyle initial=false, invalidate=ViewportRule
+
+# Platform closed captions state
+platformClosedCaptionsEnabled initial=false

[philipj_slow, 2015/03/24 07:11:58]

Thank you! The bit of the spec where I hoped to use this says "If the user has
expressed an interest in having a track from candidates enabled based on its
text track kind, text track language, and text track label, then set its text
track mode to showing."
(https://html.spec.whatwg.org/#perform-automatic-text-track-selection)

This is what I would need to implement the above:

# The preferred kind of text track to enable automatically, either "captions",
"subtitles" or the empty string if only default tracks should be enabled
automatically.
textTrackKind type=String
# The preferred text track language. If set to the empty string, the
userPreferredLanguages() (see platform/Language.h) is used instead.
textTrackLanguage type=String

Having spelled it out, I see that this isn't what you need in this CL, using
settings.textTrackKind() != "" as a condition in StyleResolver will look
mysterious. platformClosedCaptionsEnabled does too, it'll only make sense if you
know what the menu structure looks like on Android...

Renaming the setting to textTrackIgnoreAuthorStyle or
textTrackAnyStyleOverridden would make it more accurate, but I think your
original approach seems better. The simplest thing I can come up with without
introducing another setting is to ignore all author style if any of these
settings are set. If you think more granular filtering is worthwhile I won't
object, but it will require more tests.

[srivats, 2015/03/25 15:03:28]

I removed platformClosedCaptionsEnabled and I'm ignoring author settings when
any of the user settings are present in StyleResolver. Please let me know if
this is better.

+# User style overrides for captions and subtitles
+textTrackBackgroundColor type=String
+textTrackFontFamily type=String
+textTrackFontStyle type=String
+textTrackFontVariant type=String
+textTrackTextColor type=String
+textTrackTextShadow type=String
+textTrackTextSize type=String