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

cc/layer.cc

Index: cc/layer.cc
diff --git a/cc/layer.cc b/cc/layer.cc
index 56ae606163cfbbefb1a97a1fa50ac7e713a5635c..76520348a9ec8c2adaaa46810902ced8e94a3e5d 100644
--- a/cc/layer.cc
+++ b/cc/layer.cc
@@ -452,6 +452,15 @@ void Layer::setMaxScrollOffset(gfx::Vector2d maxScrollOffset)
     setNeedsCommit();
 }
 
+void Layer::setVisibleScrollArea(gfx::Size visibleScrollArea)
+{
+    if (m_visibleScrollArea == visibleScrollArea)
+        return;
+    m_visibleScrollArea = visibleScrollArea;
+    setNeedsCommit();
+}
+
+
 void Layer::setScrollable(bool scrollable)
 {
     if (m_scrollable == scrollable)
@@ -613,6 +622,7 @@ void Layer::pushPropertiesTo(LayerImpl* layer)
     layer->setScrollable(m_scrollable);
     layer->setScrollOffset(m_scrollOffset);
     layer->setMaxScrollOffset(m_maxScrollOffset);
+    layer->setVisibleScrollArea(m_visibleScrollArea);
 
     // If the main thread commits multiple times before the impl thread actually draws, then damage tracking
     // will become incorrect if we simply clobber the updateRect here. The LayerImpl's updateRect needs to