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It's unclear how much the "Start Speaking" on the whole
page is used. It was implemented only three years ago and
has never worked correctly. It's not clear what it even
means to speak the entire web document for modern web
pages. However, this makes a decent attempt at doing so.

BUG=659753, 584798, 585164, 819773
TEST=as in 659753

Change-Id: I252f38c7c7879173c3c4e0afd9dc3a42c81b8a64
Reviewed-on: https://chromium-review.googlesource.com/956029Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543920}