Move the panel container above the shelf.

The callout widgets (arrows) pointing down from the panel windows need to show above the shelf and must be stacked with the panel windows in order for the shadows to be drawn correctly. A new container is created for shelf bubble windows (currently only the overflow bubble) so that it can show above the attached panel windows.

BUG=292840
TEST=Open a panel window such as hangouts and then maximize a browser window. The arrow pointing to the launcher icon from the panel window should still be visible.

Review URL: https://codereview.chromium.org/30673003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230125 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230125