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This fixes a bug where in gmail selecting text in an open draft also
selected text behind that pop up window. The bug was occuring because
the hittest was returning a node that was behind the open dialog/window,
and select-to-speak walked up its tree to find a higher level root with
which to build the nodes to speak.

The bug only showed up on high density screens because then DIP and
PX were significantly different. On 1:1 screens it could not be
found.

A test is added which fails without this one-line change but passes
with it.

Bug: 792545
Change-Id: I21519b6cfaae97cb11c711b11b7a6b3e4b6fe017
Reviewed-on: https://chromium-review.googlesource.com/826043
Commit-Queue: Katie D <katie@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523992}