Smaller assets for phantom windows when using --ash-enable-alternate-caption-button (Code change in https://codereview.chromium.org/147103003/)

BUG=335443
TEST=None
TBR=skuhne, jamescook

Review URL: https://codereview.chromium.org/145483004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248531 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248531