Bug 735612 - Add missing synchronization around viewport manipulation. r=Cwiiis

mobile/android/base/ui/PanZoomController.java

@@ -232,8 +232,10 @@ public class PanZoomController
         case NOTHING:
             // Don't do animations here; they're distracting and can cause flashes on page
             // transitions.
+            synchronized (mController) {
                 mController.setViewportMetrics(getValidViewportMetrics());
                 mController.notifyLayerClientOfGeometryChange();
+            }
             break;
         }
     }
@@ -241,6 +243,7 @@ public class PanZoomController
     /** This must be called on the UI thread. */
     public void pageSizeUpdated() {
         if (mState == PanZoomState.NOTHING) {
+            synchronized (mController) {
                 ViewportMetrics validated = getValidViewportMetrics();
                 if (! (new ViewportMetrics(mController.getViewportMetrics())).fuzzyEquals(validated)) {
                     // page size changed such that we are now in overscroll. snap to the
@@ -250,6 +253,7 @@ public class PanZoomController
                 }
             }
         }
+    }
 
     /*
      * Panning/scrolling