Fix a bug where window.screen would return 0, 0 as the width and height

Fix a bug where window.screen would return 0, 0 as the width and height
if the page requesting the dimensions didn't have a GDK window associated
with it and there was no GDK window under the mouse pointer.

Instead of using gdk_display_get_window_at_pointer, use
gdk_display_get_default_group which will always return a valid value.  This
means extension background pages will always get the screen size of the
primary monitor.

BUG=86605
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=90231

[Daniel Erat, 2011-06-22 22:40:41]

LGTM

[tony, 2011-06-22 22:44:25]

+jam for content approval

[jam, 2011-06-23 18:05:01]

lgtm