Implement one-page-at-a-time mousewheel scrolling in the impl thread.

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index cddbec75d02a5a56574fb679414dd5b457445962..a7e6685a03357e6fa4267ee59bb67e25d5240e07 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -1255,7 +1255,8 @@ static gfx::Vector2dF scrollLayerWithLocalDelta(LayerImpl& layerImpl, gfx::Vecto
 }
 
 bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
-                                 const gfx::Vector2d& scrollDelta)
+                                 const gfx::Vector2d& scrollDelta,
+                                 bool scrollByPage)
 {
     TRACE_EVENT0("cc", "LayerTreeHostImpl::scrollBy");
     if (!currentlyScrollingLayer())
@@ -1272,6 +1273,10 @@ bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
         if (m_settings.pageScalePinchZoomEnabled && layerImpl == rootScrollLayer())
             viewport = &m_pinchZoomViewport;
         gfx::Vector2dF appliedDelta;
+        gfx::Size visibleScrollArea = layerImpl->visibleScrollArea();
+        if (scrollByPage)
+            pendingDelta = gfx::ScaleVector2d(pendingDelta, visibleScrollArea.width(), visibleScrollArea.height());

[jamesr, 2013/03/06 23:50:37]

This seems a bit strange.  scrollByPage is only valid for mousewheels and wheels
can't be diagonal.

+
         if (m_topControlsManager && layerImpl == rootScrollLayer())
             pendingDelta = m_topControlsManager->ScrollBy(pendingDelta);
 
@@ -1281,6 +1286,13 @@ bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
         } else
             appliedDelta = scrollLayerWithLocalDelta(*layerImpl, pendingDelta);
 
+        if (scrollByPage) {
+            if (visibleScrollArea.width() != 0.0)
+                appliedDelta.set_x(appliedDelta.x() / visibleScrollArea.width());
+            if (visibleScrollArea.height() != 0.0)
+                appliedDelta.set_y(appliedDelta.y() / visibleScrollArea.height());
+        }
+
         // If the layer wasn't able to move, try the next one in the hierarchy.
         float moveThresholdSquared = 0.1f * 0.1f;
         if (appliedDelta.LengthSquared() < moveThresholdSquared)