Bug 764188 - Expose a global frame tree generation number. r=dbaron

This generation number will be incremented every time that the frame
tree for a presshell changes. Callers need to deal with the case where
this value overflows. We use this to avoid a bunch of Android
screenshotting.

widget/android/nsAppShell.cpp

@@ -24,6 +24,7 @@
 #include "nsIDOMClientRect.h"
 #include "nsIDOMWakeLockListener.h"
 #include "nsIPowerManagerService.h"
+#include "nsFrameManager.h"
 
 #include "mozilla/Services.h"
 #include "mozilla/unused.h"
@@ -117,6 +118,15 @@ class AfterPaintListener : public nsIDOMEventListener {
     }
 
     virtual nsresult HandleEvent(nsIDOMEvent* aEvent) {
+        PRUint32 generation = nsFrameManager::GetGlobalGenerationNumber();
+        if (mLastGeneration == generation) {
+            // the frame tree has not changed since our last AfterPaint
+            // so we can drop this event.
+            return NS_OK;
+        }
+
+        mLastGeneration = generation;
+
         nsCOMPtr<nsIDOMNotifyPaintEvent> paintEvent = do_QueryInterface(aEvent);
         if (!paintEvent)
             return NS_OK;
@@ -139,6 +149,7 @@ class AfterPaintListener : public nsIDOMEventListener {
     }
 
   private:
+    PRUint32 mLastGeneration;
     nsCOMPtr<nsIDOMEventTarget> mEventTarget;
 };