Makes PlatformEventSource creation in Env conditional

Makes PlatformEventSource creation in  Env conditional

Currently mojo runs all services in the same process. We have separate
services that manage the display and one that runs aura. The display
one needs to create PlatformEventSource (as it gets events). The
service running aura doesn't need a PlatformEventSource as it gets
events from the display one. This means I need the ability to create
aura::Env without a PlatformEventSource.

I believe the long term plan is to move some services out of
process. When this happens this boolean shouldn't be needed.

BUG=365012
TEST=none
R=sadrul@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=268148

[sadrul, 2014-05-03 01:38:16]

A couple of notes: (1) All calls to Env::CreateInstance() in this CL passes
true. I assume the view-manager will pass false in a future CL? (2) I noticed
this morning that the aura-demo example mojo-app fails to run. My fix for that
was to remove the event-source it attempts to create. I don't know if the same
fix would apply for the view-manager.

Also, no-idea-dog to some extent!

rs lgtm

[sky, 2014-05-03 04:44:23]

Having view-manager pass false to this is in a separate patch. There were other
bugs that vtl recently fixed likely effected mojo-aura, but I haven't tried it
recently myself.

[sky, 2014-05-03 04:44:28]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2014-05-03 04:44:32]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sky@chromium.org/262883011/1

[commit-bot: I haz the power, 2014-05-03 04:48:45]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-03 04:48:45]

Try jobs failed on following builders:
  linux_chromium_gn_rel on tryserver.chromium

[sky, 2014-05-05 04:26:51]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2014-05-05 04:27:01]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sky@chromium.org/262883011/1

[commit-bot: I haz the power, 2014-05-05 06:44:20]

Change committed as 268148