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They used to be children of the root.  This change makes
shadows track their windows' transforms and simplifies the
code (as we no longer need to track stacking or parenting
changes).  I am still slightly worried that shadows will
look strange if we scale windows up or down, but we can
figure out how to deal with that when we need to.

BUG=101977
TEST=added; also hacked up StackingController to rotate and scale windows and checked that shadows were drawn as expected

Review URL: http://codereview.chromium.org/8785005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@112823 0039d316-1c4b-4281-b951-d872f2087c98