cc: Fix comment about update rect. This rect is in layer space.

R=enne
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/103163002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238518 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238518