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

cc/scrollbar_layer.cc

Index: cc/scrollbar_layer.cc
diff --git a/cc/scrollbar_layer.cc b/cc/scrollbar_layer.cc
index 3d83aebf30f541d6036e683f2ef32d9363dd2fa7..3d1a9f82f61fc9f692ed9b44a64fdc33f775a77c 100644
--- a/cc/scrollbar_layer.cc
+++ b/cc/scrollbar_layer.cc
@@ -19,11 +19,17 @@ namespace cc {
 
 scoped_ptr<LayerImpl> ScrollbarLayer::CreateLayerImpl(
     LayerTreeImpl* tree_impl) {
-  return ScrollbarLayerImpl::Create(
-      tree_impl,
-      id(),
-      ScrollbarGeometryFixedThumb::create(make_scoped_ptr(geometry_->clone())))
-      .PassAs<LayerImpl>();
+  scoped_ptr<LayerImpl> scroll_layer_impl =
+      ScrollbarLayerImpl::Create(
+          tree_impl,
+          id(),
+          ScrollbarGeometryFixedThumb::create(
+              make_scoped_ptr(geometry_->clone()))).PassAs<LayerImpl>();
+
+  if (scroll_layer_id_ == Layer::PINCH_ZOOM_ROOT_SCROLL_LAYER_ID)
+    scroll_layer_impl->ToScrollbarLayer()->SetOpacity(0);

[enne (OOO), 2013/03/12 21:10:08]

Why are opacity updates blocked and this is required?

[wjmaclean, 2013/03/12 22:02:22]

The call to "ToScrollbarLayer()" is not necessary, but 

scroll_layer_impl->SetOpacity(0);

is. If you look at Layer::PushPropertiesTo(LayerImpl* layer)

you find

if (!OpacityIsAnimating())
  layer->SetOpacity(opacity_);
  
which is required for animations to work properly. If we piggyback on this then
we need to at least make sure the PZ ScrollbarLayerImpl starts with opacity = 0.

+
+  return scroll_layer_impl.Pass();
 }
 
 scoped_refptr<ScrollbarLayer> ScrollbarLayer::Create(
@@ -61,6 +67,11 @@ void ScrollbarLayer::SetScrollLayerId(int id) {
   SetNeedsFullTreeSync();
 }
 
+bool ScrollbarLayer::OpacityIsAnimating() const {
+  return Layer::OpacityIsAnimating() ||

[enne (OOO), 2013/03/12 21:10:08]

Maybe just return Layer::OpacityIsAnimating || isOverlay() here, since we'll
need this for OSX overlay scrollbars too.

[wjmaclean, 2013/03/12 22:02:22]

Sure, though Clank also uses overlay scrollbars, and I don't know how this might
affect them.

+         scroll_layer_id_ == Layer::PINCH_ZOOM_ROOT_SCROLL_LAYER_ID;
+}
+
 WebKit::WebScrollbar::Orientation ScrollbarLayer::Orientation() const {
   return scrollbar_->orientation();
 }