Fix a couple UI problems of ChromeOS BubbleWindow.
Fix a couple UI problems of ChromeOS BubbleWindow.

Set BubbleWindow's underlying GtkWidgets back pixmap to NULL to avoid
the background being painted when window gets mapped. This is just a
workaround. For the right fix, we need to figure out why chrome triggers
gdk_window_configure_finished being called prematurely (i.e. before we
finished at least one paint.)

Also enabling double buffering for BubbleWindow per chromium-os:9065.

BUG=chromium-os:8538,chromium-os:9065
TEST=Verify fix for chromium-os:8535 and chromium-os:9065.

Review URL: http://codereview.chromium.org/4590002

git-svn-id: http://src.chromium.org/svn/trunk/src@66282 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 4659ba37dfa9be777163a75b5c81eefc3329f388
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser