[GTK] Don't go back twice when double-clicking the mouse's back button.
[GTK] Don't go back twice when double-clicking the mouse's back button.

BUG=45789
TEST=see bug

Review URL: http://codereview.chromium.org/2676002

git-svn-id: http://src.chromium.org/svn/trunk/src@49216 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 4871423f8cd357485a3f3ae46d969914c5db4424
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