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 507e35233d988931dda147a16fd5a75dc327b567..51ce55105125b7153c20bfb5fb40557d6ed68a1c 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -1198,6 +1198,7 @@ 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;
         }
 
@@ -1211,6 +1212,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         if (status == ScrollOnMainThread) {
             m_numMainThreadScrolls++;
             UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
+            activeTree()->DidBeginScroll();
             return ScrollOnMainThread;
         }
 
@@ -1233,6 +1235,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         m_numImplThreadScrolls++;
         m_client->renewTreePriority();
         UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", false);
+        activeTree()->DidBeginScroll();
         return ScrollStarted;
     }
     return ScrollIgnored;
@@ -1356,6 +1359,7 @@ bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
             break;
     }
 
+    activeTree()->DidUpdateScroll();
     if (didScroll) {
         m_client->setNeedsCommitOnImplThread();
         m_client->setNeedsRedrawOnImplThread();
@@ -1375,6 +1379,7 @@ void LayerTreeHostImpl::scrollEnd()
     if (m_topControlsManager)
         m_topControlsManager->ScrollEnd();
     clearCurrentlyScrollingLayer();
+    activeTree()->DidEndScroll();
 }
 
 void LayerTreeHostImpl::pinchGestureBegin()