Bug 1343027 - 2. Add jni::Ref::Lock function; r=snorp

Add a Lock function to jni::Ref to lock the referenced object akin to
the synchronized keyword in Java. It returns an AutoLock RAII object
that automatically unlocks the object when going out of scope.

widget/android/jni/Refs.h

@@ -79,6 +79,45 @@ protected:
 public:
     using JNIType = Type;
 
+    class AutoLock
+    {
+        friend class Ref<Cls, Type>;
+
+        JNIEnv* const mEnv;
+        Type mInstance;
+
+        AutoLock(Type aInstance)
+            : mEnv(FindEnv())
+            , mInstance(mEnv->NewLocalRef(aInstance))
+        {
+            mEnv->MonitorEnter(mInstance);
+            MOZ_CATCH_JNI_EXCEPTION(mEnv);
+        }
+
+    public:
+        AutoLock(AutoLock&& aOther)
+            : mEnv(aOther.mEnv)
+            , mInstance(aOther.mInstance)
+        {
+            aOther.mInstance = nullptr;
+        }
+
+        ~AutoLock()
+        {
+            Unlock();
+        }
+
+        void Unlock()
+        {
+            if (mInstance) {
+                mEnv->MonitorExit(mInstance);
+                mEnv->DeleteLocalRef(mInstance);
+                MOZ_CATCH_JNI_EXCEPTION(mEnv);
+                mInstance = nullptr;
+            }
+        }
+    };
+
     // Construct a Ref form a raw JNI reference.
     static Ref<Cls, Type> From(JNIType obj)
     {
@@ -117,6 +156,11 @@ public:
         return T::Ref::From(*this);
     }
 
+    AutoLock Lock() const
+    {
+        return AutoLock(mInstance);
+    }
+
     bool operator==(const Ref& other) const
     {
         // Treat two references of the same object as being the same.

widget/android/nsWindow.cpp

@@ -784,12 +784,9 @@ nsWindow::AndroidView::GetSettings(JSContext* aCx, JS::MutableHandleValue aOut)
         return NS_OK;
     }
 
-    JNIEnv* const env = jni::GetGeckoThreadEnv();
-    env->MonitorEnter(mSettings.Get());
-    nsresult rv = widget::EventDispatcher::UnboxBundle(aCx, mSettings, aOut);
-    env->MonitorExit(mSettings.Get());
-
-    return rv;
+    // Lock to prevent races with UI thread.
+    auto lock = mSettings.Lock();
+    return widget::EventDispatcher::UnboxBundle(aCx, mSettings, aOut);
 }
 
 /**