Bug 1411692 - move forward declaration to Refs.h; r=darchons

clang complains if we don't have a declaration for Context::sClassRef;
Accessors.h had one, but it's not in the right place to prevent clang
from complaining.  Moving it closer to where Context itself is declared
seems like a reasonable thing to do.

widget/android/jni/Accessors.h

@@ -264,10 +264,6 @@ public:
 template<class T> jfieldID Field<T>::sID;
 
 
-// Define the sClassRef member declared in Refs.h and
-// used by Method and Field above.
-template<class C, typename T> jclass Context<C, T>::sClassRef;
-
 } // namespace jni
 } // namespace mozilla
 

widget/android/jni/Refs.h

@@ -295,6 +295,7 @@ public:
     }
 };
 
+template<class C, typename T> jclass Context<C, T>::sClassRef;
 
 template<class Cls, typename Type = jobject>
 class ObjectBase