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As we may have two envs, WidgetFocusManager needs some context to know which
WidgetFocusManager to return.

BUG=874554
TEST=covered by tests

Change-Id: I397e14496351fc9839273aa7284c55cdb35555c8
Reviewed-on: https://chromium-review.googlesource.com/1212088Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589593}