Bug 854289 - Fix offsetting of viewport with dynamic toolbar. r=kats

Make sure to offset the viewport *after* setting the Gecko viewport on
setFirstPaintViewport callbacks, so that we don't store an incorrect viewport
origin and end up offsetting events incorrectly.

Similarly, on page size updates, the Java-side viewport metrics are used to
update the Gecko metrics, so make sure they're clamped so that they aren't
incorrect during overscroll.

mobile/android/base/gfx/GeckoLayerClient.java

@@ -383,7 +383,9 @@ public class GeckoLayerClient implements LayerView.Listener, PanZoomTarget
                 break;
             }
 
-            final ImmutableViewportMetrics geckoMetrics = newMetrics;
+            // Update the Gecko-side viewport metrics. Make sure to do this
+            // before modifying the metrics below.
+            final ImmutableViewportMetrics geckoMetrics = newMetrics.clamp();
             post(new Runnable() {
                 @Override
                 public void run() {
@@ -591,12 +593,6 @@ public class GeckoLayerClient implements LayerView.Listener, PanZoomTarget
         synchronized (this) {
             ImmutableViewportMetrics currentMetrics = getViewportMetrics();
 
-            // If we're meant to be scrolled to the top, take into account any
-            // margin set on the pan zoom controller.
-            if (FloatUtils.fuzzyEquals(offsetY, pageTop)) {
-                offsetY = -currentMetrics.fixedLayerMarginTop;
-            }
-
             final ImmutableViewportMetrics newMetrics = currentMetrics
                 .setViewportOrigin(offsetX, offsetY)
                 .setZoomFactor(zoom)
@@ -612,7 +608,16 @@ public class GeckoLayerClient implements LayerView.Listener, PanZoomTarget
                     mGeckoViewport = newMetrics;
                 }
             });
-            setViewportMetrics(newMetrics);
+
+            // If we're meant to be scrolled to the top, take into account any
+            // margin set on the pan zoom controller.
+            if (FloatUtils.fuzzyEquals(offsetY, pageTop)
+                  && newMetrics.fixedLayerMarginTop > 0) {
+                setViewportMetrics(newMetrics.setViewportOrigin(offsetX,
+                    -newMetrics.fixedLayerMarginTop));
+            } else {
+                setViewportMetrics(newMetrics);
+            }
 
             Tab tab = Tabs.getInstance().getSelectedTab();
             mView.setBackgroundColor(tab.getBackgroundColor());