Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Currently RecordingImageBufferSurface determines whether
the canvas was cleared by tracking opaque draws.  For this
purpose we need all draw operations that completely
overwrite the destination pixels to be treated as opaque
even though the resulting pixels may not be opaque
(alpha = 1). To solve this problem we introduce an alternate
mode of operation in OpaqueRegionSkia, which was renamed to
RegionTracker, to reflect its new more generic vocation.

BUG=397205
TEST=blink platform unit test RecordingImageBufferSurface.clearRect

Review URL: https://codereview.chromium.org/413313002

git-svn-id: svn://svn.chromium.org/blink/trunk@179074 bbb929c8-8fbe-4397-9dbb-9b2b20218538