Add initial GTK web accessibility framework (new with valgrind fix).

Add initial GTK web accessibility framework (new with valgrind fix).

This is the same as http://codereview.chromium.org/9839069/ which was
landed but reverted due to a valgrind failure in content_unittests.

The valgrind failure was because browser_accessibility_manager_unittest.cc
is constructing its own subclass of BrowserAccessibility (for testing),
but BrowserAccessibilityManagerGtk was assuming that any BrowserAccessibility
could be static_casted to a BrowserAccessibilityGtk. I fixed this by
making ToBrowserAccessibilityGtk() a virtual method with a default impl that
returns NULL.

Original change:

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,sky