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- backward selections were previously not implemented/supported. Add that feature in this change.
- suppress hints during selection.

chromevox_tests --gtest_filter=BackgroundTest.Selection

Test: manually select forwards and backwards in all possible granularities.
Bug: 854872
Change-Id: Ic26a9e2d1426e8fcea31323c33c42c3476b1bc4c
Reviewed-on: https://chromium-review.googlesource.com/1115913Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570865}