cc: Reuse the same TaskGraph and completed tasks vector.

This improves performance by avoiding unnecessary resizing of
vectors. RasterWorkerPoolPerfTest.ScheduleTasks is ~25% faster
for ImageRasterWorkerPool with this change.

BUG=246546

Review URL: https://codereview.chromium.org/144463012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250305 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250305