Revert 242116 "Use a single ToplevelWindowEventHandler for the a..."

> Use a single ToplevelWindowEventHandler for the ash::Shell. This is part 1 in disabling the few cases in which dragging multiple windows is possible (which causes crashiness)
> 
> BUG=324038
> TEST=None
> 
> Review URL: https://codereview.chromium.org/114643003

TBR=pkotwicz@chromium.org

Review URL: https://codereview.chromium.org/115693009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242198 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242198