Provide a vsync notification to the renderer

This patch allows the renderer to subscribe to a vsync notification from
the browser. The renderer will use this notification to align its
rendering to the start of a display refresh interval to maximize the
changes a new frame is ready by the next vsync. Some platforms such as
Android also deliver input events immediately before the vsync signal,
so rendering as a response to this signal guarantees all input events
for the current frame have been received.
    
The notification will be sent for all future vsyncs until it is
explicitly disabled. This reduces IPC overhead since the notification
only needs to be requested once.
    
In the renderer the vsync notification is delivered as a callback from
the compositor output surface directly to the impl thread to avoid
unnecessary thread hops.
    
BUG=168459
TEST=LayerTreeHostTestVSyncNotification

[Sami, 2013-01-09 20:20:12]

This patch lets renderers register for vsync notification and provides the
plumbing to send the signal back. It also adds a vsync provider into Android
Content Shell for testing.

Let me know if you think this is unwieldy and should be split. Also, this
depends on https://codereview.chromium.org/11414308/.

[Sami, 2013-01-17 14:23:30]

Okay, I took the hint and reduced this to just the renderer parts and added a cc
unit test. Looks better, or WDYT?

N.B. still depends on https://codereview.chromium.org/11414308/.