Adds scroll position/scale emulation to DevTools.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 053ae8028000cff6ff1c0d18d8a862866f76b094..190716778a92be08b827ad10e4c7f2993ae88e12 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2548,6 +2548,12 @@ LayerImpl* LayerTreeHostImpl::FindScrollLayerForDeviceViewportPoint(
   *main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
 
+  // Falling back to the root scroll layer ensures generation of root overscroll
+  // notifications. The inner viewport layer represents the viewport during
+  // scrolling.
+  if (!layer_impl)
+    layer_impl = InnerViewportScrollLayer();
+
   // Walk up the hierarchy and look for a scrollable layer.
   LayerImpl* potentially_scrolling_layer_impl = NULL;
   if (layer_impl) {
@@ -2580,12 +2586,6 @@ LayerImpl* LayerTreeHostImpl::FindScrollLayerForDeviceViewportPoint(
       }
     }
   }
-  // Falling back to the root scroll layer ensures generation of root overscroll
-  // notifications while preventing scroll updates from being unintentionally
-  // forwarded to the main thread. The inner viewport layer represents the
-  // viewport during scrolling.
-  if (!potentially_scrolling_layer_impl)
-    potentially_scrolling_layer_impl = InnerViewportScrollLayer();
 
   // The inner viewport layer represents the viewport.
   if (potentially_scrolling_layer_impl == OuterViewportScrollLayer())