--- a/gfx/layers/apz/util/ChromeProcessController.cpp
+++ b/gfx/layers/apz/util/ChromeProcessController.cpp
@@ -92,8 +92,36 @@ ChromeProcessController::Destroy()
       FROM_HERE,
       NewRunnableMethod(this, &ChromeProcessController::Destroy));
     return;
   }
 
   MOZ_ASSERT(MessageLoop::current() == mUILoop);
   mWidget = nullptr;
 }
+
+float
+ChromeProcessController::GetPresShellResolution() const
+{
+  // The document in the chrome process cannot be zoomed, so its pres shell
+  // resolution is 1.
+  return 1.0f;
+}
+
+void
+ChromeProcessController::HandleSingleTap(const CSSPoint& aPoint,
+                                         int32_t aModifiers,
+                                         const ScrollableLayerGuid& aGuid)
+{
+  if (MessageLoop::current() != mUILoop) {
+    mUILoop->PostTask(
+        FROM_HERE,
+        NewRunnableMethod(this, &ChromeProcessController::HandleSingleTap,
+                          aPoint, aModifiers, aGuid));
+    return;
+  }
+
+  LayoutDevicePoint point =
+      APZCCallbackHelper::ApplyCallbackTransform(aPoint, aGuid, GetPresShellResolution())
+    * mWidget->GetDefaultScale();
+
+  APZCCallbackHelper::FireSingleTapEvent(point, mWidget);
+}

--- a/gfx/layers/apz/util/ChromeProcessController.h
+++ b/gfx/layers/apz/util/ChromeProcessController.h
@@ -33,28 +33,29 @@ public:
   virtual void RequestContentRepaint(const FrameMetrics& aFrameMetrics) MOZ_OVERRIDE;
   virtual void PostDelayedTask(Task* aTask, int aDelayMs) MOZ_OVERRIDE;
   virtual void AcknowledgeScrollUpdate(const FrameMetrics::ViewID& aScrollId,
                                        const uint32_t& aScrollGeneration) MOZ_OVERRIDE;
 
   virtual void HandleDoubleTap(const mozilla::CSSPoint& aPoint, int32_t aModifiers,
                                const ScrollableLayerGuid& aGuid) MOZ_OVERRIDE {}
   virtual void HandleSingleTap(const mozilla::CSSPoint& aPoint, int32_t aModifiers,
-                               const ScrollableLayerGuid& aGuid) MOZ_OVERRIDE {}
+                               const ScrollableLayerGuid& aGuid) MOZ_OVERRIDE;
   virtual void HandleLongTap(const mozilla::CSSPoint& aPoint, int32_t aModifiers,
                                const ScrollableLayerGuid& aGuid,
                                uint64_t aInputBlockId) MOZ_OVERRIDE {}
   virtual void HandleLongTapUp(const CSSPoint& aPoint, int32_t aModifiers,
                                const ScrollableLayerGuid& aGuid) MOZ_OVERRIDE {}
   virtual void SendAsyncScrollDOMEvent(bool aIsRoot, const mozilla::CSSRect &aContentRect,
                                        const mozilla::CSSSize &aScrollableSize) MOZ_OVERRIDE {}
 
 private:
   nsCOMPtr<nsIWidget> mWidget;
   MessageLoop* mUILoop;
 
   void InitializeRoot();
+  float GetPresShellResolution() const;
 };
 
 } // namespace layers
 } // namespace mozilla
 
 #endif /* mozilla_layers_ChromeProcessController_h */