cc: Prevent integer overflow with software TextureMailbox.

      Perform a CHECK() when creating a software TextureMailbox since these
come from outside of cc's control.

Do unsigned integer multiplication to produce an unsigned result,
preventing overflow when possible.

R=piman@chromium.org
BUG=348332

Review URL: https://codereview.chromium.org/220093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260969 0039d316-1c4b-4281-b951-d872f2087c98