Bug 1187552 - Support direct ownership of C++ objects by Java objects; r=snorp

Add a direct ownership model where the Java object owns the
corresponding C++ object directly, in addition to the WeakPtr model
where the Java object owns a WeakPtr to the C++ object. The WeakPtr
model is chosen when the implementing C++ class inherits from
SupportsWeakPtr. Otherwise, the direct ownership model is chosen. Under
the direct ownership model, a UniquePtr object must be used to attach
the containing C++ object to a Java object, to ensure ownership is
passed on to the Java object.

widget/android/jni/Utils.cpp

@@ -111,14 +111,8 @@ uintptr_t GetNativeHandle(JNIEnv* env, jobject instance)
         return 0;
     }
 
-    auto handle = static_cast<uintptr_t>(
+    return static_cast<uintptr_t>(
             env->GetLongField(instance, sJNIObjectHandleField));
-
-    if (!handle && !env->ExceptionCheck()) {
-        ThrowException(env, "java/lang/NullPointerException",
-                       "Null native pointer");
-    }
-    return handle;
 }
 
 void SetNativeHandle(JNIEnv* env, jobject instance, uintptr_t handle)