Revert "Add gpu::MailboxHolder to hold state for a gpu::Mailbox"

This reverts commit 9ee2343406a6fae9c8aacc7303f7b1e5a04aab9e.

This patch is the source of the top crash in canary. See the bug
for more details.

TBR=sheu
NOTREECHECKS=true
BUG=336040

Review URL: https://codereview.chromium.org/129873005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245959 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245959