Fix a crasher with "select file" dialogs.

A crasher would sometimes happen when opening a selet file dialog
(such as when loading an unpacled extension).
This is because we would reinterpret cast the user data of the HWND
to a WidgetWin, and in that case the Windows dialog has a user-data
that we did not set.
This CL checks that the HWND is at least a WindowImpl before attempting
to retrieve the WidgetWin, ensuring we won't mess with windows we did
not create directly.

BUG=44312
TEST=Open the extension page. Move the focus by pressing TAB to the
     "Load unpacked extension..." button. Press ENTER many times quickly
     so it opens/closes the "Select dir" dialog. It should not crash.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=47672

[Ben Goodger (Google), 2010-05-19 00:00:16]

OK