Bug 892246 - Allow subdocument scrolling to reveal margins. r=kats

This adds a notification callback to PanZoomTarget that the PanZoomController
can call to notify GeckoLayerClient that a subdocument is being scrolled. This
allows GeckoLayerClient to call LayerMarginsAnimator and alter the margins
accordingly, stopping a page from trapping the toolbar on/off the screen with
a screen-covering subframe.

mobile/android/base/gfx/GeckoLayerClient.java

@@ -854,6 +854,21 @@ public class GeckoLayerClient implements LayerView.Listener, PanZoomTarget
         viewportMetricsChanged(true);
     }
 
+    /** Implementation of PanZoomTarget
+     * Notification that a subdocument has been scrolled by a certain amount.
+     * This is used here to make sure that the margins are still accessible
+     * during subdocument scrolling.
+     *
+     * You must hold the monitor while calling this.
+     */
+    @Override
+    public void onSubdocumentScrollBy(float dx, float dy) {
+        ImmutableViewportMetrics newMarginsMetrics =
+            mMarginsAnimator.scrollBy(mViewportMetrics, dx, dy);
+        mViewportMetrics = mViewportMetrics.setMarginsFrom(newMarginsMetrics);
+        viewportMetricsChanged(true);
+    }
+
     /** Implementation of PanZoomTarget */
     @Override
     public void panZoomStopped() {

mobile/android/base/gfx/JavaPanZoomController.java

@@ -806,7 +806,11 @@ class JavaPanZoomController
         if (FloatUtils.fuzzyEquals(displacement.x, 0.0f) && FloatUtils.fuzzyEquals(displacement.y, 0.0f)) {
             return;
         }
-        if (! mSubscroller.scrollBy(displacement)) {
+        if (mSubscroller.scrollBy(displacement)) {
+            synchronized (mTarget.getLock()) {
+                mTarget.onSubdocumentScrollBy(displacement.x, displacement.y);
+            }
+        } else {
             synchronized (mTarget.getLock()) {
                 scrollBy(displacement.x, displacement.y);
             }

mobile/android/base/gfx/PanZoomTarget.java

@@ -17,6 +17,7 @@ public interface PanZoomTarget {
     public void setAnimationTarget(ImmutableViewportMetrics viewport);
     public void setViewportMetrics(ImmutableViewportMetrics viewport);
     public void scrollBy(float dx, float dy);
+    public void onSubdocumentScrollBy(float dx, float dy);
     public void panZoomStopped();
     /** This triggers an (asynchronous) viewport update/redraw. */
     public void forceRedraw(DisplayPortMetrics displayPort);