Fix (or tag) uses of explicit '64' instead of GL_MAILBOX_SIZE_CHROMIUM

That way we can easily change the size later. Security says 64 may be overkill.

BUG=None

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=251306

R=danakj@chromium.org

Review URL: https://codereview.chromium.org/138763009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251453 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251453