cc: Update invalidation perftest to clean up released tiles.

The tiles are actually being correctly cleaned up at the end of
the test, but it seems that it accumulates quite a few of them
and times out at the end. This patch ensures that all of the tiles
are cleaned up after each invalidate run.

R=reveman@chromium.org
BUG=298999

Review URL: https://codereview.chromium.org/24487007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225666 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225666