Bug 1343027 - 1. Add jni::Ref::Cast function; r=snorp

Add a Cast function to jni::Ref to cast a reference to a different type
(e.g. if ref is a jni::Object::Ref referring to a String instance, use |
ref.Cast<jni::String>()->Length() | to get the length of the string).

In MOZ_CHECK_JNI builds, Cast() checks that an instance of the source
class can indeed be cast to the target class.

widget/android/jni/Refs.h

@@ -106,6 +106,17 @@ public:
                 mInstance, typename T::Context().ClassRef());
     }
 
+    template<class T>
+    typename T::Ref Cast() const
+    {
+#ifdef MOZ_CHECK_JNI
+        MOZ_RELEASE_ASSERT(FindEnv()->IsAssignableFrom(
+                Context<Cls, Type>().ClassRef(),
+                typename T::Context().ClassRef()));
+#endif
+        return T::Ref::From(*this);
+    }
+
     bool operator==(const Ref& other) const
     {
         // Treat two references of the same object as being the same.