[UI->Navigation] Forward navigation button issue with onbeforeunload dialog

[UI->Navigation] Forward navigation button issue with onbeforeunload dialog

Forward navigation button on toolbar gets enabled even though there
is unload confirmation dialog and it remians in enabled state even
after dialog box is closed with "Stay"
There are two possible fixes
1> Do not enable the forward navigation button till user accept the
   navigation with "Leave"
2> Disable it back when user cancel the navigation with "Stay"

This patchset is based on the option two mentioned above

BUG=603805
TEST=Login to http://demo.redmine.org site with some credentials.
(1) Go to any issue link present in http://demo.redmine.org/issues/
(2) Click the Edit link -> Text area box will appear
(3) Put some characters in the text box
(4) Press back button from the toolbar
(5) modal dialog to confirm navigate back will appear and forward navigation button will be enabled
(6) Click "Stay on this Page" button
(7) Verify that the forward navigation button is disabled after the dialog is closed

R=rnimmagadda@chromium.org