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Platform painting should be through skia::ScopedPlatformPaint.
(Only one compositor callsite was calling Begin/End directly.)

Remove skia::EndPlatformPaint() and all device-specific implementations,
which were effectively noops.

Remove skia::BeginPlatformPaint(), and make all device-specific impls
protected.

BUG=598695
R=fmalita@chromium.org
TBR=danakj@chromium.org

Review URL: https://codereview.chromium.org/1839113002

Cr-Commit-Position: refs/heads/master@{#383820}