Add support for new accessibility roles from WebKit, with tests.

content/test/data/accessibility/canvas-expected-win.txt

+ROLE_SYSTEM_DOCUMENT READONLY FOCUSABLE
+    IA2_ROLE_SECTION READONLY
+        ROLE_SYSTEM_GRAPHIC

[aboxhall, 2012/09/11 04:24:42]

So read-only canvas doesn't show up as canvas role?

[dmazzoni, 2012/09/11 09:14:42]

Yeah, the idea is that some people treat canvas just like an image, i.e.

  <canvas alt="User icon" width=128 height=128><canvas>

In this case, it might make sense for the screen reader to call it a "graphic"
or "image" with alt text. For example, there's often a key to jump to the next
graphic or provide your own label for a graphic, or perform OCR on a graphic -
but nothing equivalent for a canvas. But when it has interactive content, it's a
group for screen readers that don't understand the Canvas role, otherwise it's a
canvas.

+        IA2_ROLE_CANVAS
+            ROLE_SYSTEM_LINK FOCUSABLE
+                ROLE_SYSTEM_TEXT