Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

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 cc928ce8429a5c2480db6d3ca7fe44465ff761e5..091cdea4dd01f4fee88b92f0cbd7883db62057a4 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -1203,6 +1203,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         if (layerImpl->TryScroll(deviceViewportPoint, type) == ScrollOnMainThread) {
             m_numMainThreadScrolls++;
             UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
+            activeTree()->DidBeginScroll();
             return ScrollOnMainThread;
         }
 
@@ -1216,6 +1217,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         if (status == ScrollOnMainThread) {
             m_numMainThreadScrolls++;
             UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
+            activeTree()->DidBeginScroll();
             return ScrollOnMainThread;
         }
 
@@ -1238,6 +1240,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         m_numImplThreadScrolls++;
         m_client->renewTreePriority();
         UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", false);
+        activeTree()->DidBeginScroll();
         return ScrollStarted;
     }
     return ScrollIgnored;
@@ -1361,6 +1364,7 @@ bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
             break;
     }
 
+    activeTree()->DidUpdateScroll();
     if (didScroll) {
         m_client->setNeedsCommitOnImplThread();
         m_client->setNeedsRedrawOnImplThread();
@@ -1380,6 +1384,7 @@ void LayerTreeHostImpl::scrollEnd()
     if (m_topControlsManager)
         m_topControlsManager->ScrollEnd();
     clearCurrentlyScrollingLayer();
+    activeTree()->DidEndScroll();
 }
 
 void LayerTreeHostImpl::pinchGestureBegin()