Allow masks to be created from one image.

Modify image_tools to allow lists of one image to be used
to create a mask.

BUG=None
NOTRY=True

Review URL: https://codereview.chromium.org/98563005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238995 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238995