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..b3c8bc68313e1530a382346134dea353fc6614e4 100644
--- a/Source/core/frame/Settings.in
+++ b/Source/core/frame/Settings.in
@@ -325,3 +325,12 @@ 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
+
+# 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

[philipj_slow, 2015/03/17 02:22:31]

What is the expected format of this string? In the Chromium CL it's a
percentage, if that's the only intended usage maybe this should be a type=double
initial=1.0 with some asserts?

[srivats, 2015/03/18 01:58:13]

Font size css property can have multiple formats like percentage,
small/medium/large, length(px) etc. On Android, we get font-size from FontScale
where 1 corresponds to 100% and medium. The Chromium-side CL converts the
FontScale into a percentage but it could also be represented as
small/medium/large. Leaving this as String gives the flexibility to set it in
different formats.

[philipj_slow, 2015/03/18 06:31:09]

OK, so the actual data type of the setting is secondary. The important thing is
what this actually should do. On Android, is the system-level style the only
involved, or how does the size setting interact with formats that have internal
size settings?

It seems to me that if a user sets font scale 1.5 and that always results in
captions being rendered at (say) 30px, then that should map to forcing the font
size to be 30px for WebVTT. However, this would require distinguishing font
scale 1 from a "untouched default" font scale, but does the Android API
differentiate between these?