aura: Require explicit ownership of the Env instance.

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}

[sadrul, 2015-08-18 05:36:44]

sadrul@chromium.org changed reviewers:
+ sky@chromium.org

[sky, 2015-08-18 20:35:43]

LGTM

[sadrul, 2015-08-18 20:50:20]

sadrul@chromium.org changed reviewers:
+ ben@chromium.org

[sadrul, 2015-08-18 20:50:21]

+ben@ for top-level owner of chromecast/ content/shell and content/browser
changes for the updated API (will TBR)

[sadrul, 2015-08-18 21:21:29]

The CQ bit was checked by sadrul@chromium.org

[commit-bot: I haz the power, 2015-08-18 21:21:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1293013002/120001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1293013002/120001

[commit-bot: I haz the power, 2015-08-18 21:27:25]

Committed patchset #7 (id:120001)

[commit-bot: I haz the power, 2015-08-18 21:28:16]

Patchset 7 (id:??) landed as
https://crrev.com/535b77f94d63736b21583c76e2dbf1f2a75e6377
Cr-Commit-Position: refs/heads/master@{#344037}

[Fady Samuel, 2015-08-18 22:44:59]

A revert of this CL (patchset #7 id:120001) has been created in
https://codereview.chromium.org/1297173003/ by fsamuel@chromium.org.

The reason for reverting is: This broke mandoline --enable-multiprocess on
Linux:

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