Clarify the byte ordering and premultiplied-ness of the Pepper 2D data.

BUG=none
TEST=none (no code change)
Review URL: http://codereview.chromium.org/524074

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@35918 0039d316-1c4b-4281-b951-d872f2087c98