move SkMessageBus::Get out of header

src/core/SkMessageBus.h

Index: src/core/SkMessageBus.h
diff --git a/src/core/SkMessageBus.h b/src/core/SkMessageBus.h
index 0a408319c15ef197c4cfbf02398d7168e1cae813..ddeac57ac12d2ac5ff6b9e871eb555167cfbb601 100644
--- a/src/core/SkMessageBus.h
+++ b/src/core/SkMessageBus.h
@@ -44,6 +44,18 @@ private:
     SkMutex           fInboxesMutex;
 };
 
+// This must go in a single .cpp file, not some .h, or we risk creating more than one global
+// SkMessageBus per type when using shared libraries.
+#define DECLARE_SKMESSAGEBUS_MESSAGE(Message)             \
+    template <>                                           \
+    SkMessageBus<Message>* SkMessageBus<Message>::Get() { \
+        static SkMessageBus<Message>* bus = NULL;         \
+        SK_DECLARE_STATIC_ONCE(once);                     \
+        SkOnce(&once, &New, &bus);                        \
+        SkASSERT(bus != NULL);                            \
+        return bus;                                       \
+    }
+
 //   ----------------------- Implementation of SkMessageBus::Inbox -----------------------
 
 template<typename Message>
@@ -93,17 +105,6 @@ template <typename Message>
 }
 
 template <typename Message>
-/*static*/ SkMessageBus<Message>* SkMessageBus<Message>::Get() {
-    // The first time this method is called, create the singleton bus for this message type.
-    static SkMessageBus<Message>* bus = NULL;
-    SK_DECLARE_STATIC_ONCE(once);
-    SkOnce(&once, &New, &bus);
-
-    SkASSERT(bus != NULL);
-    return bus;
-}
-
-template <typename Message>
 /*static*/ void SkMessageBus<Message>::Post(const Message& m) {
     SkMessageBus<Message>* bus = SkMessageBus<Message>::Get();
     SkAutoMutexAcquire lock(bus->fInboxesMutex);