Image assets for phantom windows when using --ash-enable-alternate-caption-button

BUG=328930
TEST=None
TBR=sky

Review URL: https://codereview.chromium.org/134963004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244790 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244790