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Also avoids unnecessary heap allocations by recycling old state vector.
Result is ~100% performance improvement to ScheduleTasks.

BUG=269841
TEST=cc_perftests --gtest_filter=RasterWorkerPoolPerfTests/RasterWorkerPoolPerfTest.ScheduleTasks/0

Review URL: https://codereview.chromium.org/174453003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255615 0039d316-1c4b-4281-b951-d872f2087c98