[Profile picker] Set accessible window title based on content title

This CL implements accessibe window title for the picker widget based
on the title of its WebContents. In order to get meaningful titles, the
CL also implements dynamic updating of the html title for the
chrome://profile-picker WebUI app.

Bug: 1171376
Change-Id: Iad1fbf2b3f1610a80d65faa43bfd7002249fbc9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2652894
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Monica Basta <msalama@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848498}