Move Serial declaration to Forward.h

This is a bit ugly but avoids having to include all of SerialQueue.h
just to get this typedef.

src/backend/common/Forward.h

@@ -63,6 +63,8 @@ namespace backend {
     template<typename T>
     class PerStage;
 
+    using Serial = uint64_t;
+
     // TODO(cwallez@chromium.org): where should constants live?
     static constexpr uint32_t kMaxPushConstants = 32u;
     static constexpr uint32_t kMaxBindGroups = 4u;

src/backend/common/SerialQueue.h

@@ -22,8 +22,6 @@
 
 namespace backend {
 
-    using Serial = uint64_t;
-
     template<typename T>
     class SerialQueue {
         private: