New media playback UI.

Source/core/frame/Settings.in

 # Defines properties which are available on the Settings object.
 #
 # Please think carefully before adding a new Setting.  Some questions to
 # consider are:
 # - Should this be a RuntimeEnabledFeature instead? Settings are for things
 #   which we support either values of at runtime.  Features are set at renderer
 #   process startup and are never changed. Features also tend to be set to a
 #   value based on the platform or the stability of the code in question, where
 #   as settings both codepaths need to be stable.
 # - How will you ensure test coverage of all relevant values of your setting?
@@ skipping 333 matching lines @@
 textTrackFontFamily type=String
 textTrackFontStyle type=String
 textTrackFontVariant type=String
 textTrackTextColor type=String
 textTrackTextShadow type=String
 textTrackTextSize type=String
 
 lowPriorityIframes initial=false
 
 reportWheelOverscroll initial=false
+
+# Do we want to try to save screen real estate in the media player by hiding
+# the volume slider?  It is not guaranteed that the volume slider will always
+# be hidden if this is set.
+preferHiddenVolumeSlider initial=false

[philipj_slow, 2015/07/10 14:51:58]

Could these be collapsed into a single setting, after all I suppose they will
always be in sync.

[liberato (no reviews please), 2015/07/10 15:32:19]

i did that originally, but separated them for reasons that are no longer clear
to me after a few hours sleep.

i'll merge.

+
+# Do we want to try to save screen real estate in the media player by hiding
+# the mute button when possible?
+preferHiddenMuteButton initial=false
+
+# The media player will only hide volume / mute if the corresponding
+# prefer* setting is on, and if it's using the new UI.  This setting lets
+# us override the second part, and enable it for the previous UI as well.
+# TODO(liberato): remove this setting once the old UI is removed.  It
+# facilitates testing without worrying about which UI is enabled.
+forceAllowHiddenAudioElements initial=false

[philipj_slow, 2015/07/10 14:21:35]

Setting that are for testing only can be put in
Source/core/testing/InternalSettings.*

testing/Internals.idl is also an option, you could use that to reach into
MediaPlayer and tickle it directly, but it doesn't matter which way it's done I
think.

[liberato (no reviews please), 2015/07/10 15:32:19]

that's cool!  I moved it to Internals.idl, since that's ten shades of awesome.