Makes PlatformEventSource creation in Env conditional

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 9f19b1487e0db581ca01e84cf72919b9fce59ff8..8bd446080263993aff58d53d189a3da8867f02c1 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -828,7 +828,7 @@ void Shell::Init() {
   // Env creates the compositor. Historically it seems to have been implicitly
   // initialized first by the ActivationController, but now that FocusController
   // no longer does this we need to do it explicitly.
-  aura::Env::CreateInstance();
+  aura::Env::CreateInstance(true);
 
   // The WindowModalityController needs to be at the front of the input event
   // pretarget handler list to ensure that it processes input events when modal