Use WINDOW instead of BUBBLE for the throbber widget.

This fix solves the problem with the throbber widget is always shown on
the top of the windows.

Also it uses parent GtkWindow instead of GtkWidget.

BUG=chromium-os:9770
TEST=manaual

Review URL: http://codereview.chromium.org/5964004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@70192 0039d316-1c4b-4281-b951-d872f2087c98