aura: makes each Window be associated with an Env

This is to avoid using Env::GetInstance(), so that there can be multiple
Envs created at once.

BUG=847992
TEST=covered by tests

Change-Id: I2e2604c8f8a356ca9cc6771ed050ef8431e7b840
Reviewed-on: https://chromium-review.googlesource.com/1165861
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581568}
20 files changed