Make IsSolidColor() a property on CC scrollbar layers.

cc/layers/scrollbar_layer.cc

Index: cc/layers/scrollbar_layer.cc
diff --git a/cc/layers/scrollbar_layer.cc b/cc/layers/scrollbar_layer.cc
index 30e2a93ba5bf453eba0b0d1bcff9965f9dfeacab..d3c8ded34c0084377c3d620d4a35f07e09c5703b 100644
--- a/cc/layers/scrollbar_layer.cc
+++ b/cc/layers/scrollbar_layer.cc
@@ -21,21 +21,26 @@ namespace cc {
 scoped_ptr<LayerImpl> ScrollbarLayer::CreateLayerImpl(
     LayerTreeImpl* tree_impl) {
   return ScrollbarLayerImpl::Create(
-      tree_impl, id(), scrollbar_->Orientation()).PassAs<LayerImpl>();
+      tree_impl, id(), scrollbar_->Orientation(), is_solid_color_)
+      .PassAs<LayerImpl>();
 }
 
 scoped_refptr<ScrollbarLayer> ScrollbarLayer::Create(
     scoped_ptr<Scrollbar> scrollbar,
-    int scroll_layer_id) {
+    int scroll_layer_id,
+    bool is_solid_color) {
   return make_scoped_refptr(new ScrollbarLayer(scrollbar.Pass(),
-                                               scroll_layer_id));
+                                               scroll_layer_id,
+                                               is_solid_color));
 }
 
 ScrollbarLayer::ScrollbarLayer(
     scoped_ptr<Scrollbar> scrollbar,
-    int scroll_layer_id)
+    int scroll_layer_id,
+    bool is_solid_color)
     : scrollbar_(scrollbar.Pass()),
       scroll_layer_id_(scroll_layer_id),
+      is_solid_color_(is_solid_color),
       texture_format_(GL_INVALID_ENUM) {
   if (!scrollbar_->IsOverlay())
     SetShouldScrollOnMainThread(true);
@@ -59,13 +64,19 @@ ScrollbarOrientation ScrollbarLayer::Orientation() const {
   return scrollbar_->Orientation();
 }
 
+bool ScrollbarLayer::IsSolidColor() const {
+  DCHECK(layer_tree_host());

[enne (OOO), 2013/06/26 17:39:05]

This seems dangerous.  cc::Layers can be detached from their host at any point.

What about having WebScrollbarLayerImpl check the command line for this force
flag and pass that to the ScrollbarLayer constructor, in the same way that we do
for making WebContentLayerImpl decide to be backed by ContentLayer vs.
PictureLayer?

[wjmaclean, 2013/06/26 18:16:00]

There isn't a command line flag for force_solid_color_scrollbars. It only exists
as a layer tree setting, meaning that the value of this could (theoretically)
differ between RenderWidgetCompositor instances.

My original design tried to get around this by retaining a distinction between
(1) the is_solid_color_ value the scrollbar was created with, and (2) the
settings of the currently attached layer_tree_host_, and or-ing them to
determine what to do.

WebScrollbarLayerImpl's default to false then unless overriden by the settings,
and ScrollbarLayers created to be pinch-inner-viewport scrollbars have this
value set at construction time.

Could we go back to that?

I'm assuming that WebScrollbarLayerImpl shouldn't know about which compositor
instance will use it until it's attached to a tree, is that right?

[enne (OOO), 2013/06/26 20:02:11]

Right now it can't differ, and is always true on Android:
https://code.google.com/p/chromium/codesearch#chromium/src/content/renderer/g...

I'm not sure allowing it to differ per compositor is a feature we should
consider supporting.  I suspect moving a ScrollbarLayer between hosts with
different solid color scrollbars probably would behave quite poorly.

+  return is_solid_color_ ||
+         layer_tree_host()->settings().force_solid_color_scrollbars;
+}
+
 int ScrollbarLayer::MaxTextureSize() {
   DCHECK(layer_tree_host());
   return layer_tree_host()->GetRendererCapabilities().max_texture_size;
 }
 
 float ScrollbarLayer::ClampScaleToMaxTextureSize(float scale) {
-  if (layer_tree_host()->settings().solid_color_scrollbars)
+  if (IsSolidColor())
     return scale;
 
   // If the scaled content_bounds() is bigger than the max texture size of the
@@ -104,8 +115,7 @@ void ScrollbarLayer::PushPropertiesTo(LayerImpl* layer) {
 
   ScrollbarLayerImpl* scrollbar_layer = static_cast<ScrollbarLayerImpl*>(layer);
 
-  if (layer_tree_host() &&
-      layer_tree_host()->settings().solid_color_scrollbars) {
+  if (layer_tree_host() && IsSolidColor()) {
     int thickness_override =
         layer_tree_host()->settings().solid_color_scrollbar_thickness_dip;
     if (thickness_override != -1) {
@@ -127,6 +137,7 @@ void ScrollbarLayer::PushPropertiesTo(LayerImpl* layer) {
     scrollbar_layer->set_track_start(track_rect_.y());
     scrollbar_layer->set_track_length(track_rect_.height());
   }
+  scrollbar_layer->set_is_solid_color(IsSolidColor());
 
   if (track_ && track_->texture()->have_backing_texture())
     scrollbar_layer->set_track_resource_id(track_->texture()->resource_id());
@@ -174,7 +185,7 @@ class ScrollbarPartPainter : public LayerPainter {
 };
 
 void ScrollbarLayer::CreateUpdaterIfNeeded() {
-  if (layer_tree_host()->settings().solid_color_scrollbars)
+  if (IsSolidColor())
     return;
 
   texture_format_ =
@@ -212,7 +223,7 @@ void ScrollbarLayer::UpdatePart(CachingBitmapContentLayerUpdater* painter,
                                 gfx::Rect rect,
                                 ResourceUpdateQueue* queue,
                                 RenderingStats* stats) {
-  if (layer_tree_host()->settings().solid_color_scrollbars)
+  if (IsSolidColor())
     return;
 
   // Skip painting and uploading if there are no invalidations and
@@ -267,7 +278,7 @@ gfx::Rect ScrollbarLayer::ScrollbarLayerRectToContentRect(
 
 void ScrollbarLayer::SetTexturePriorities(
     const PriorityCalculator& priority_calc) {
-  if (layer_tree_host()->settings().solid_color_scrollbars)
+  if (IsSolidColor())
     return;
 
   if (content_bounds().IsEmpty())
@@ -296,7 +307,7 @@ void ScrollbarLayer::Update(ResourceUpdateQueue* queue,
                             RenderingStats* stats) {
   track_rect_ = scrollbar_->TrackRect();
 
-  if (layer_tree_host()->settings().solid_color_scrollbars)
+  if (IsSolidColor())
     return;
 
   {