GitLab

Adds the IDL code and accessor methods into ComputedAccessibleNode for
the intersection of properties between AccessibleNode and
AXStringAttribute. These accessors will return null strings to the
window in a few cases:
   - when the AXNode for a given matching AXID is not in the tree,
   - when the string property has not been set.

Currently, the accessors are rudimentary and require an enum
translation, although this may change in future in favour of an
individual hard coded method on the AomContentAXTree.

BUG=792783

Change-Id: Icd9cac2cb8486d549932f400b0aeeae569a1f712
Reviewed-on: https://chromium-review.googlesource.com/883863
Commit-Queue: Meredith Lane <meredithl@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533834}