Previously reviewed here:
https://codereview.chromium.org/60353008/

Mac-specific implementation of the GlobalShortcutListener class that
listens for global shortcuts. Handles basic keyboard intercepting and
forwards its output to the base class for processing.

Adds two things:
1. Intercepts media keys. Uses an event tap for intercepting media keys
(PlayPause, NextTrack, PreviousTrack).
2. Binds keyboard shortcuts (hot keys). Carbon RegisterEventHotKey API for
binding to non-media key global hot keys (eg. Command-Shift-1).

Mac tests added but disabled while we figure out flakiness it causes in another test.

BUG=302437

Review URL: https://codereview.chromium.org/105713005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241864 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241864