Remove deferred clear from SkGpuDevice

Add combining to GrClearBatch

Fix issue with state tracking in GrGLGpu::createTestingOnlyBackendTexture

Add tests for clearing GPU SkSurfaces and add tests for GrDrawContext::clear().

Add comment that SkCanvas::flush will resolve the RT in the GPU case.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1658823002

Review URL: https://codereview.chromium.org/1658823002
12 files changed