Tab audio mute control (views UI), behind a switch (off by default).

This change consolidates all of the existing media indicator UI into a
new MediaIndicatorButton class.  When the indicator is transitioned to
the audio playing or muting state, the button functionality activates;
and, when clicked, will toggle tab-wide audio muting.  Otherwise, the
view only serves to paint the indicator icon with all mouse/gesture
events handled by Tab (its parent View).

Added UMA's to detect usage of the feature, attempted usage of the
disabled feature, and possible close button mis-clicks.

This new feature is off by default.  The plan is to run a UI experiment
and gather the data needed to drive later launch decisions.

Note: This change depends on:

TEST=With --enable-tab-audio-muting, user can click audio indicator to toggle tab muting, or use the tab context menu to toggle muting of one or more selected tabs.

Review URL:

Cr-Commit-Position: refs/heads/master@{#296869}
30 files changed