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This significantly reduces the context switching overhead for
impl-side painting.

Instead of posting reply task to the impl thread after completing
each raster job, post a reply only when worker pool becomes idle
and poll for completed tasks at a 6ms interval.

This doesn't just make rasterization more efficient but also
reduces the number of shallow flushes, which makes async uploads
more efficient.

BUG=173802

Review URL: https://codereview.chromium.org/12217105

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@182404 0039d316-1c4b-4281-b951-d872f2087c98