Call PlatformExit when closing all windows in Shell.

In the views implementation when shell_views.cc is closed it deletes the
views::Widget which will delete DesktopNativeWidgetAura and, in turn it will
delete the compositor.

With toolkit_views=0 shell_aura.cc is responsible for creating and destroying
the ui::Compositor. Currently the compositor is deleted on PlatformExit which
is done after the message loop finishes. However when the shell is closing it
will schedule a draw update. Since PlatformExit is executed too late, the
compositor is still around and will try to perform the update.

TEST=Ran content_browsertests
BUG=315392

Review URL: https://codereview.chromium.org/83943004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237607 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237607