cc: Add mailbox support to ResourceProvider write locks.

This adds support for mailboxes to ScopedWriteLockGL. Using the mailbox
requires using ScopedTextureProvider/ScopedSkSurfaceProvider which
ensures that the texture id for the mailbox is destroyed after use on
the worker context.

This CL also includes the following cleanup:
1. ResourceProvider locks don't keep resource pointers around.
2. ScopedSamplerGL does not inherit from ScopedReadLockGL.
3. GpuRasterizer is folded back into GpuRasterBufferProvider.
4. TileTaskManager does not own RasterBufferProvider.


Cr-Commit-Position: refs/heads/master@{#398204}
28 files changed