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As part of the mash work we're going to have multiple aura::Envs at the same
time. Code that may run in Ash now gets the Env from aura::Window, rather than
using Env's static GetInstance(). This patch converts exo to remember the
Env so that it can get the right one.

BUG=847992
TEST=covered by tests

Change-Id: I5fd1c6120868b3847ccd68a063039eec5b54a3b6
Reviewed-on: https://chromium-review.googlesource.com/1169590
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582303}