Revert 135020 - Add initial GTK web accessibility framework.

Revert 135020 - Add initial GTK web accessibility framework.

This enables Linux desktop assistive technology such as the Orca screen
reader to access the web contents. Builds on the same accessibility stack
used by Mac & Windows already.

This change works with Orca now, but it's minimal: all that works is getting
the correct feedback when you tab through focusable links and controls in
a webpage. Future changes will add the rest of the support.

BUG=24585
TEST=Run Chrome on desktop Linux with Orca turned on, tab through links.
TBR=darin


Review URL: http://codereview.chromium.org/9839069

TBR=dmazzoni@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=135027