Make IsSolidColor() a property on CC scrollbar layers.

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 03d0100be421c881d100ff3ea908541d0d383dbd..4dd9bbdc670fc604d33bb8175b16003ef34a8513 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -223,7 +223,13 @@ void LayerTreeImpl::UpdateMaxScrollOffset() {
 }
 
 void LayerTreeImpl::UpdateSolidColorScrollbars() {
-  DCHECK(settings().solid_color_scrollbars);
+  // TODO(wjmaclean) This logic will need to change when we re-factor
+  // RootScrollLayer() into InnerViewportScrollLayer() and
+  // OuterViewportScrollLayer(), since in the desktop case the
+  // OuterViewportScrollLayer() won't have solid color
+  // scrollbars, while the InnerViewportScrollLayer() will. Ultimately, the
+  // plan is for this function to disappear altogether.
+  DCHECK(settings().force_solid_color_scrollbars);
 
   LayerImpl* root_scroll = RootScrollLayer();
   DCHECK(root_scroll);
@@ -252,7 +258,7 @@ void LayerTreeImpl::UpdateDrawProperties() {
   if (IsActiveTree() && RootScrollLayer() && RootClipLayer())
     UpdateRootScrollLayerSizeDelta();
 
-  if (settings().solid_color_scrollbars &&
+  if (settings().force_solid_color_scrollbars &&
       IsActiveTree() &&
       RootScrollLayer()) {
     UpdateSolidColorScrollbars();