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 f959d8ec41e4a855031c51d2ab5fa71fcccecec9..f7ce350540f2ac1078429b7c9a1478c67c806174 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -223,7 +223,12 @@ void LayerTreeImpl::UpdateMaxScrollOffset() {
 }
 
 void LayerTreeImpl::UpdateSolidColorScrollbars() {
-  DCHECK(settings().solid_color_scrollbars);
+  // FIXME: 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 +257,7 @@ void LayerTreeImpl::UpdateDrawProperties() {
   if (IsActiveTree() && RootScrollLayer() && RootClipLayer())
     UpdateRootScrollLayerSizeDelta();
 
-  if (settings().solid_color_scrollbars &&
+  if (settings().force_solid_color_scrollbars &&
       IsActiveTree() &&
       RootScrollLayer()) {
     UpdateSolidColorScrollbars();