Bug 869940 - Implement pan start/end notifications from APZC. r=kats

gfx/layers/ipc/AsyncPanZoomController.cpp

@@ -1378,9 +1378,24 @@ void AsyncPanZoomController::ContentReceivedTouch(bool aPreventDefault) {
   }
 }
 
-void AsyncPanZoomController::SetState(PanZoomState aState) {
-  MonitorAutoLock monitor(mMonitor);
-  mState = aState;
+void AsyncPanZoomController::SetState(PanZoomState aNewState) {
+
+  PanZoomState oldState;
+
+  // Intentional scoping for mutex
+  {
+    MonitorAutoLock monitor(mMonitor);
+    oldState = mState;
+    mState = aNewState;
+  }
+
+  if (mGeckoContentController) {
+    if (oldState == PANNING && aNewState != PANNING) {
+      mGeckoContentController->HandlePanEnd();
+    } else if (oldState != PANNING && aNewState == PANNING) {
+      mGeckoContentController->HandlePanBegin();
+    }
+  }
 }
 
 void AsyncPanZoomController::TimeoutTouchListeners() {

gfx/layers/ipc/GeckoContentController.h

@@ -60,6 +60,17 @@ public:
    */
   virtual void PostDelayedTask(Task* aTask, int aDelayMs) = 0;
 
+
+  /**
+   * Request any special actions be performed when panning starts
+   */
+  virtual void HandlePanBegin() {}
+
+  /**
+   * Request any special actions be performed when panning ends
+   */
+  virtual void HandlePanEnd() {}
+
   GeckoContentController() {}
   virtual ~GeckoContentController() {}
 };