Re-apply shape specified by SetRegion in ResetWindowRegion.

This is a minimal patch to fix the AppWindow.setShape API.

BUG=324077

Review URL: https://codereview.chromium.org/95973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238577 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238577