Set region for windows that are expanded.

If windows are expanded we need to set a window region so that clicks on the transparent area fall through to whatever's below.

BUG=286609
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/75523003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235960 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235960