Fix crash in shutdown of cast_shell browser process

Fix crash in shutdown of cast_shell browser process

Ensure that aura::Env is destroyed after CastBrowserProcess's
members.  During shutdown, CastBrowserProcess destructor can end up
calling aura::Window::~Window (via application destruction and
RenderWidgetHostImpl::Destroy), which needs access to the aura::Env
instance.

BUG= internal b/21156227

Committed: https://crrev.com/d14567b9de030b52377027c368f15be0942d8259
Cr-Commit-Position: refs/heads/master@{#330107}

[halliwell, 2015-05-15 15:48:36]

halliwell@chromium.org changed reviewers:
+ gunsch@chromium.org, lcwu@chromium.org

[gunsch, 2015-05-15 15:57:20]

lgtm, but where do we create the Aura instance? Is it lazily created for us when
we call GetInstance() in CastContentWindow?

[halliwell, 2015-05-15 16:01:45]

On 2015/05/15 15:57:20, gunsch wrote:
> lgtm, but where do we create the Aura instance? Is it lazily created for us
when
> we call GetInstance() in CastContentWindow?

Yep, we rely on lazy creation.  IIRC, creating the instance creates the
compositor and leads to GPU process creation, so it's important that we don't
create it eagerly given our resource grant/revoke model.

[halliwell, 2015-05-15 16:24:06]

The CQ bit was checked by halliwell@chromium.org

[commit-bot: I haz the power, 2015-05-15 16:25:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1139823005/1

[commit-bot: I haz the power, 2015-05-15 16:36:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-15 16:37:30]

Patchset 1 (id:??) landed as
https://crrev.com/d14567b9de030b52377027c368f15be0942d8259
Cr-Commit-Position: refs/heads/master@{#330107}