[Mac] Fix the new avatar button's drop shadow in guest windows

Screenshot in the bug.

BUG=343635

Review URL: https://codereview.chromium.org/178433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253255 0039d316-1c4b-4281-b951-d872f2087c98
add suppression for handle leaks from HDCOffscreen::draw
TBR=reed@google.com
BUG=346842

Review URL: https://codereview.chromium.org/179803006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253256 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253255
SVN changes up to revision 253256