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Add an accessible window title called "Assistant" for Assistant container
view, thus ChromeVox can read the window title when invoking multilayered
shortcut:Search + a > w.

Bug: 910559
Test: local compile and manually test.
Change-Id: Ia71e08567cf448abce1e9a5d59b9fe50f34be8a2
Reviewed-on: https://chromium-review.googlesource.com/c/1357650
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613291}