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

This reverts commit c179496f2b0ebc20360350c65aff0c4941c57e26.

Reason for revert: The test failure was fixed at https://chromium-review.googlesource.com/c/chromium/src/+/1168466 . The failure was a lingering issue and not caused by patch (but triggered by my patch), so reverting the revert.

Original change's description:
> 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}

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

Change-Id: I83e61d6ca9de8dcd023757f03bb18f0abf09790e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 847992
Reviewed-on: https://chromium-review.googlesource.com/1168565
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581796}
20 files changed