Add viewport scrollbar class to support overlay scrollbars for pinch zoom virtual viewport.

webkit/renderer/compositor_bindings/web_scrollbar_layer_impl.cc

Index: webkit/renderer/compositor_bindings/web_scrollbar_layer_impl.cc
diff --git a/webkit/renderer/compositor_bindings/web_scrollbar_layer_impl.cc b/webkit/renderer/compositor_bindings/web_scrollbar_layer_impl.cc
index a8080212beb7ba55e548044a884ba70ec6fa86ad..f1e68aac472e532b4eff970cc59978b14c9aff5d 100644
--- a/webkit/renderer/compositor_bindings/web_scrollbar_layer_impl.cc
+++ b/webkit/renderer/compositor_bindings/web_scrollbar_layer_impl.cc
@@ -4,12 +4,12 @@
 
 #include "webkit/renderer/compositor_bindings/web_scrollbar_layer_impl.h"
 
+#include "cc/input/viewport_scrollbar.h"
 #include "cc/layers/scrollbar_layer.h"
-#include "third_party/WebKit/public/platform/WebScrollbar.h"
 #include "webkit/renderer/compositor_bindings/scrollbar_impl.h"
-#include "webkit/renderer/compositor_bindings/web_layer_impl.h"
 
 using cc::ScrollbarLayer;
+using cc::ViewportScrollbar;
 
 namespace webkit {
 
@@ -23,6 +23,22 @@ WebScrollbarLayerImpl::WebScrollbarLayerImpl(
               painter,
               make_scoped_ptr(geometry))).Pass(), 0))) {}
 
+WebScrollbarLayerImpl::WebScrollbarLayerImpl(

[enne (OOO), 2013/06/12 19:44:38]

I am not really sure this is in line with what I was expecting.  I was hoping we
could be in a state where all scrollbars behaved identically rather than having
two different kinds of scrollbars.

Is this some intermediate state on the way to that goal?

[wjmaclean, 2013/06/12 19:54:36]

By "behave identically" you mean using cc:Scrollbars using Blink
geometry/painters? Or do you mean full-fledged Blink Scrollbar-objects with
their own ScrollableAreas, etc.

The latter seems hard if we stay with the "Blink-core doesn't know about the
viewports" philosophy, though I could be wrong.

[trchen, 2013/06/12 21:22:11]

I agree with enne@ that we should avoid adding new CC layer type unless
necessary. However I want to support wjmaclean@ on this one. ScrollbarLayer that
uses Blink-core ScrollbarTheme to paint has always been a headache in the past.
We actually needed to make a quick-hack scrollbar back in M25 age. It just
worked. Simple, fast and clean.

Also the pinch zoom scrollbar has very different behavior than regular
scrollbars -- its thumb resizes all the time during pinch gesture.
ScrollbarLayer won't suffice if later we want to paint fancy thumb textures that
can't be simply stretched.

At this point I can't think of a reason to create full-fledged ScrollableArea.
One good reason to implement ScrollableArea is to receive
GraphicsLayer::didScroll() notification (which is weird, why didn't we simply
add a API to GraphicsLayerClient?), but for the pinch zoom viewport it's better
to use the special-cased WebViewImpl::applyScrollAndScale.

+    WebKit::WebLayer* scroll_layer,
+    WebKit::WebScrollbar::Orientation orientation,
+    size_t thickness)
+    : layer_(new WebLayerImpl(ScrollbarLayer::Create(
+          scoped_ptr<cc::Scrollbar>(new ViewportScrollbar(
+              static_cast<WebLayerImpl*>(scroll_layer)->layer(),
+              orientation == WebKit::WebScrollbar::Horizontal ? cc::HORIZONTAL
+                                                              : cc::VERTICAL,
+              thickness)),
+          0))) {
+  // Do this here to be consistent with the layer passed to the
+  // ViewportScrollbar.
+  setScrollLayer(scroll_layer);
+}
+
 WebScrollbarLayerImpl::~WebScrollbarLayerImpl() {}
 
 WebKit::WebLayer* WebScrollbarLayerImpl::layer() { return layer_.get(); }