Simplify validation routines for writing an image to the clipboard.

BUG=319285

Review URL: https://codereview.chromium.org/80403002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237290 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237290