Make transparent windows at least 64x64 pixels on windows

The ATI drivers corrupt them when they're smaller, so expand them out to at least 64 pixels wide and tall, leaving the outside transparent.

BUG=286609
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/70623007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235513 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235513