Timeline: add support for deferred canvas rasterization

Timeline: add support for deferred canvas rasterization

- add PlatformInsutrmentation::{will,did}RasterizeCanvas()
- report the above events from Canvas2DLayerBridge::prepareMailbox()
- add TimelineAgent::{will,did}UpdateLayer, plumb it through WebDevToolsAgent;
- emit Rasterize event when canvas is being rasterized during update
    of a layer that belongs to the inspected page.

Related chromium change: https://codereview.chromium.org/16848010

BUG=244505

[pfeldman, 2013-06-13 17:30:26]

lgtm. A test?

[pfeldman, 2013-08-30 09:11:13]

What is the status of this issue?

[pfeldman, 2013-10-24 13:51:38]

This is not moving, removing myself for reviewers.