Make aura_demo work on Mac

Fix a few instances where !Windows is assumed to be Linux.

Chang many instances of !USE_GTK_MESSAGE_PUMP &&
!OS_MACOSX && !OS_ANDROID to just USE_AURA (they are
equivalent now).

Add a very minimal RootWindowHostMac.

Of note is that building with Aura requires running
"gyp_chromium --no-circular-check" (the circular
check is only enabled on Mac, and doesn't work with
Aura).

Also note that the demo does not work properly unless BGRA
support is disabled (in FeatureInfo::InitializeBasicState these
extensions can be disabled).

BUG=331669

Review URL: https://codereview.chromium.org/110283005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243392 0039d316-1c4b-4281-b951-d872f2087c98


CrOS-Libchrome-Original-Commit: 3580aaf313e8fbfbec870e76a10a2461ab3f30c0
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index f4ed5a1..04709e6 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -96,11 +96,11 @@
 class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
  public:
 
-#if defined(USE_GTK_MESSAGE_PUMP)
-  typedef MessagePumpGdkObserver Observer;
-#elif !defined(OS_MACOSX) && !defined(OS_ANDROID)
+#if defined(USE_AURA)
   typedef MessagePumpDispatcher Dispatcher;
   typedef MessagePumpObserver Observer;
+#elif defined(USE_GTK_MESSAGE_PUMP)
+  typedef MessagePumpGdkObserver Observer;
 #endif
 
   // A MessageLoop has a particular type, which indicates the set of