Revert 50784 - Canvas refactoring part 3....

Revert 50784 - Canvas refactoring part 3.

- Replace Canvas instance users with CanvasSkia users.
- Rename Canvas2 to Canvas.
- Delete Canvas subclass of CanvasSkia.

This has created some ugliness around the fact that people that used SkCanvas methods on Canvas now have to go through AsCanvasSkia first. This is temporary ugliness that will be eradicated as I incrementally build out the new Canvas API.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/2825018

TBR=ben@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=50790