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We're going to change to use more than one background windows to enclose panels in the stack because not all panels are fully aligned when some of them are being dragged. The 'Window' suffix is added to the naming to make it clear.

BUG=223155
TEST=none due to no functionality change
TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/14188014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194666 0039d316-1c4b-4281-b951-d872f2087c98