Revert of aura: Require explicit ownership of the Env instance. (patchset #7 id:12000...

Revert of aura: Require explicit ownership of the Env instance. (patchset #7 id:120001 of https://codereview.chromium.org/1293013002/ )

Reason for revert:
This broke mandoline --enable-multiprocess on Linux:

[0818/183133:FATAL:x11_event_source.cc(80)] Check failed: display_.

Original issue's description:
> aura: Require explicit ownership of the Env instance.
>
> Notable changes:
> . Change aura::Env::CreateInstance() to return a scoped_ptr<> to the aura::Env instance,
>   so that there's a clear owner of the instance, and random code can no longer destroy
>   it.
> . Make aura::Env::CreateInstance() fail if more than one instance is attempted to be
>   created.
> . Remove the boolean parameter to aura::Env::CreateInstance(), since that is no longer
>   necessary, and always create a PlatformEventSource instance (if one hasn't already
>   been created).
> . BrowserMainLoop takes care of creating/destroying the aura::Env instance. So the
>   various BrowserMainParts implementations no longer need to do that.
>
> BUG=none
> TBR=ben@chromium.org as top-level owner for API change updates.
>
> Committed: https://crrev.com/535b77f94d63736b21583c76e2dbf1f2a75e6377
> Cr-Commit-Position: refs/heads/master@{#344037}

TBR=sky@chromium.org,ben@chromium.org,sadrul@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=none

Review URL: https://codereview.chromium.org/1297173003

Cr-Commit-Position: refs/heads/master@{#344058}