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 775b9655f83a1fc6d59fa64ee1e9220946c0117d..4b7c22f4796d7d07a739c26381b4281c2b5cb03b 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -1359,6 +1359,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     if (status == ScrollOnMainThread) {
       num_main_thread_scrolls_++;
       UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
+      active_tree()->DidBeginScroll();
       return ScrollOnMainThread;
     }
 
@@ -1372,6 +1373,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     if (status == ScrollOnMainThread) {
       num_main_thread_scrolls_++;
       UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
+      active_tree()->DidBeginScroll();
       return ScrollOnMainThread;
     }
 
@@ -1396,6 +1398,7 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     num_impl_thread_scrolls_++;
     client_->RenewTreePriority();
     UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", false);
+    active_tree()->DidBeginScroll();
     return ScrollStarted;
   }
   return ScrollIgnored;
@@ -1553,6 +1556,7 @@ bool LayerTreeHostImpl::ScrollBy(gfx::Point viewport_point,
       break;
   }
 
+  active_tree()->DidUpdateScroll();
   if (did_scroll) {
     client_->SetNeedsCommitOnImplThread();
     client_->SetNeedsRedrawOnImplThread();
@@ -1570,6 +1574,7 @@ void LayerTreeHostImpl::ScrollEnd() {
   if (top_controls_manager_)
     top_controls_manager_->ScrollEnd();
   ClearCurrentlyScrollingLayer();
+  active_tree()->DidEndScroll();
 }
 
 void LayerTreeHostImpl::PinchGestureBegin() {