Fix phantom windows for small windows when using --ash-enable-alternate-caption-button.

This CL:
- Uses smaller assets for drawing the phantom window. This decreases the minimum size that a phantom window can be drawn at and still look good (i.e. the left and right corner assets do not overlap)
- Enforces a minimum size for the phantom window

BUG=335443
TEST=Manual, see bug

Review URL: https://codereview.chromium.org/147103003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248581 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248581