Move Mailbox from cc to gpu, and its traits to gpu/ipc
Mailbox is safer to IPC than coaxing it into a string, because otherwise we need
to validate the size when coming from untrusted sources.
This is to enable passing Mailbox through IPC without having to depend on cc.
Among others I will need it to move Pepper to mailboxes, and pepper can't depend
on cc.
BUG=164095
Review URL: https://chromiumcodereview.appspot.com/
12378053
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@186006 0039d316-1c4b-4281-b951-d872f2087c98