Revert "aura: makes each Window be associated with an Env"

This reverts commit 99df3b2694dd3dab4860e9f20ee5a6235d9746c4.

Reason for revert: Caused failures on msan bot.

Original change's description:
> 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}

TBR=sky@chromium.org,msw@chromium.org

Change-Id: I50fb5f0d27011219ed2569b1035a78e01b9b3de9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 847992
Reviewed-on: https://chromium-review.googlesource.com/1168323
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581718}
20 files changed