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 a63973287b65d8df81016df3c452d76ad000b810..69ae854d9e40a509c22ac88f5df8796e641645ae 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -1195,6 +1195,8 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         if (layerImpl->tryScroll(deviceViewportPoint, type) == ScrollOnMainThread) {
             m_numMainThreadScrolls++;
             UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
+            if (activeTree()->HasPinchZoomScrollbars())
+                activeTree()->FadeInPinchZoomScrollbars();
             return ScrollOnMainThread;
         }
 
@@ -1208,6 +1210,8 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         if (status == ScrollOnMainThread) {
             m_numMainThreadScrolls++;
             UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", true);
+            if (activeTree()->HasPinchZoomScrollbars())
+                activeTree()->FadeInPinchZoomScrollbars();
             return ScrollOnMainThread;
         }
 
@@ -1230,6 +1234,8 @@ InputHandlerClient::ScrollStatus LayerTreeHostImpl::scrollBegin(gfx::Point viewp
         m_numImplThreadScrolls++;
         m_client->renewTreePriority();
         UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", false);
+        if (activeTree()->HasPinchZoomScrollbars())
+            activeTree()->FadeInPinchZoomScrollbars();
         return ScrollStarted;
     }
     return ScrollIgnored;
@@ -1353,6 +1359,7 @@ bool LayerTreeHostImpl::scrollBy(const gfx::Point& viewportPoint,
             break;
     }
 
+    activeTree()->UpdatePinchZoomScrollbarsIfNeeded();
     if (didScroll) {
         m_client->setNeedsCommitOnImplThread();
         m_client->setNeedsRedrawOnImplThread();
@@ -1372,6 +1379,8 @@ void LayerTreeHostImpl::scrollEnd()
     if (m_topControlsManager)
         m_topControlsManager->ScrollEnd();
     clearCurrentlyScrollingLayer();
+    if (activeTree()->HasPinchZoomScrollbars())
+        activeTree()->FadeOutPinchZoomScrollbars();
 }
 
 void LayerTreeHostImpl::pinchGestureBegin()