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I'm doing this as a precursor to having the WindowTarget know
the Window it's associated with. We could certainly support
non-WindowTargeters, but I don't think that really buys us
anything.

BUG=878836
TEST=covered by tests

Change-Id: Iabe32319ba28796d4b90e967617bc4f95ff5581e
Reviewed-on: https://chromium-review.googlesource.com/1195666Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587718}