Workaround bad bitmaps in clibpoard code

      - Some bitmaps end up with a NULL getPixels(). Don't try to copy ot of that.
- Only try to copy 32-bit bitmaps
- Protect against overflow in size computation

BUG=369621

Review URL: https://codereview.chromium.org/289573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271730 0039d316-1c4b-4281-b951-d872f2087c98