Remove const from int return type on aura::Env::mouse_button_flags()

I hit this building views_examples_exe on Mac with TOOLKIT_VIEWS=1.

error: 'const' type qualifier on return type has no effect [-Werror,-Wignored-qualifiers]

Added in r264669.

BUG=NONE
TEST=NONE
TBR=sadrul@chromium.org,hshi@chromium.org

Review URL: https://codereview.chromium.org/413303003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285461 0039d316-1c4b-4281-b951-d872f2087c98

ui/aura/env.h

@@ -42,7 +42,7 @@ class AURA_EXPORT Env : public ui::EventTarget, public base::SupportsUserData {
   void AddObserver(EnvObserver* observer);
   void RemoveObserver(EnvObserver* observer);
 
-  const int mouse_button_flags() const { return mouse_button_flags_; }
+  int mouse_button_flags() const { return mouse_button_flags_; }
   void set_mouse_button_flags(int mouse_button_flags) {
     mouse_button_flags_ = mouse_button_flags;
   }