Second attempt at moving the StackingClient to a property on the RootWindow.

This time, we allow the StackingClient to be optional. If not specified, unparented windows will just be added to the RootWindow.

BUG=none
TEST=existing automation

Review URL: http://codereview.chromium.org/8916020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@114571 0039d316-1c4b-4281-b951-d872f2087c98
20 files changed