SkiaOutputSurfaceImpl: fix sync token problem.

SkiaOutputSurfaceImpl::SubmitPaint() not always submits painting tasks
to GPU. So sync token returned by it is not always valid. Fix the
problem by renaming it to EndPaint() which will not submit tasks to GPU
anymore and not return sync token. It also adds a new method Flush()
which will submit pending tasks to GPU thread and provide a sync token.

Bug: 1139697
Change-Id: I34fdd5e325ac2425399a9fc98ffcaa6052cddc3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488302
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819421}