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There is still work to be done here, but this is a good step. The
interesting parts:
. An aura::WindowTargeter is used to ensure events target the right
  Window. In particular if one client intercepts events from another
  client.
. Custom EventHandlers are used to forward to the remote client. This
  is mildly tricky to ensure top-levels are handled correctly.

BUG=837692
TEST=covered by test

Change-Id: I58f0d23e7d7dd6c6c31f3cfbb44889588c4f8f84
Reviewed-on: https://chromium-review.googlesource.com/1054427
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558868}