Explicit initialization of aura::Env for browser shell.

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 12a547fe07126fe6a9624c2e3edb813d5dd8fa6f..6c6f07105843aa3e4e91673b0dee2b71b328fd0c 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -477,7 +477,7 @@ 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::GetInstance();
+  aura::Env::CreateInstance();
   views::corewm::FocusController* focus_controller =
       new views::corewm::FocusController(new wm::AshFocusRules);
   focus_client_.reset(focus_controller);