Update WebContents::RequestAXTreeSnapshot() to OnceCallback.

BUG=714018

Change-Id: Ic6fa05a2ebd6ca21bd1b2a349a58f7a419a3465d
Reviewed-on: https://chromium-review.googlesource.com/952991
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541881}
9 files changed