cc: Do bitmap conversion for RasterBuffer in the worker thread.

- Moved RasterBuffer from ResourceProvider to its own file.
It's now the public interface used by raster tasks.
- Added Acquire/ReleaseSkCanvas() interface into
RasterBuffer.
- Bitmap conversion is now done in raster worker thread
through RasterBuffer interface.

BUG=None

Review URL: https://codereview.chromium.org/454843002

Cr-Commit-Position: refs/heads/master@{#291910}
19 files changed