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.
-            mController.setViewportMetrics(getValidViewportMetrics());
-            mController.notifyLayerClientOfGeometryChange();
+            synchronized (mController) {
+                mController.setViewportMetrics(getValidViewportMetrics());
+                mController.notifyLayerClientOfGeometryChange();
+            }
             break;
         }
     }
@@ -241,12 +243,14 @@ public class PanZoomController
     /** This must be called on the UI thread. */
     public void pageSizeUpdated() {
         if (mState == PanZoomState.NOTHING) {
-            ViewportMetrics validated = getValidViewportMetrics();
-            if (! (new ViewportMetrics(mController.getViewportMetrics())).fuzzyEquals(validated)) {
-                // page size changed such that we are now in overscroll. snap to the
-                // the nearest valid viewport
-                mController.setViewportMetrics(validated);
-                mController.notifyLayerClientOfGeometryChange();
+            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
+                    // the nearest valid viewport
+                    mController.setViewportMetrics(validated);
+                    mController.notifyLayerClientOfGeometryChange();
+                }
             }
         }
     }