Refactor the PlatformContext layer to have only one class.

Refactor the PlatformContext layer to have only one class.

Previously we had three classes of PlatformCanvas*, one for each platform. Then
we had a typedef of PlatformContext to PlatformCanvas[Mac|Win|Linux] for the
specific platform.

This means that it was almost impossible to forward-declare PlatformCanvas and
there were a bunch of unnecessary includes of platform_canvas.h in header
files.

This change makes there be only one platform_canvas.h header with ifdefs, which
removes a decent amount of duplicated code. There is a platform-independent
file, and one platform-dependent file of platform_canvas for each platform.

I also renamed PlatformDevice[Mac|Win|Linux] to PlatformDevice, althouth in
this case I kept the separate headers since there was much less overlap.

I also broke out CanvasPaint into separate headers so this template doesn't
need to be included all over the project (only a couple of files actually need
it).

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=18363