Fix for focus traversal issue in Gears dialogs

HtmlDialogs had focus problems.
They would not be focused when shown and tab traversal was not working.

This was caused by several issues:
- HWNDView was not focusing its HWND when focused through the View::RequestFocus() method (it would rely on the default view behavior which is to focus the root view HWND), so it would not be focused by default.
- DOMView (parent of HtmlDialogView) was not focusable so would not get the focus when pressing tab.
- DOMView was not eating tab keys events (by returning false in View::CanProcessTabKeyEvents()), so the render view would not get the tab key press messages (for focus traversal in the render view).

Note: most of the file changes in this CL are just the consequence of changing the constness of the WindowDelegate::GetInitiallyFocusedView method.

BUG=6859
TEST=Open a web page, from the page menu, select the "Create application shortcut..." menu. In the dialog, press the TAB key, focus should alternate between the different fields


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=9925

[sky, 2009-02-18 00:42:48]

LGTM