Remove scroll_clip_layer dependency from viewport layer tracking

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 6ebc937dea9c6feee4c05d3befc0eef3e81ab1eb..7456229a49ea709f818c8ef4d16d39eae23e70fe 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -241,15 +241,24 @@ class CC_EXPORT LayerTreeHost : public NON_EXPORTED_BASE(SurfaceReferenceOwner),
   Layer* root_layer() { return root_layer_.get(); }
   const Layer* root_layer() const { return root_layer_.get(); }
 
-  void RegisterViewportLayers(scoped_refptr<Layer> overscroll_elasticity_layer,
-                              scoped_refptr<Layer> page_scale_layer,
-                              scoped_refptr<Layer> inner_viewport_scroll_layer,
-                              scoped_refptr<Layer> outer_viewport_scroll_layer);
+  void RegisterViewportLayers(

[wkorman, 2017/05/17 20:59:39]

For consideration -- with 6 params it's nearing point where something like a
struct with named fields that we pass in may be worthwhile for readability.
Maybe we could even just store the struct as member data then, and save a bunch
of duplicative field definitions. Maybe would simplify the INVALID_ID we have to
fall back to everywhere. Could have one which stores layers, another which
stores layer ids, and able to get one from the other.

Anyway not needed for this change but wanted to mention while reading.

Also, if the full set of these layers is going to continue to be around for a
while, is there an ascii diagram or doc we can reference with more info for
folks who want to see what all the layers are?

I'm thinking something like what we have in:

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/co...

I feel like there was another better one somewhere in Blink but not remembering
where.

[pdr., 2017/05/17 21:36:44]

Yeah I wish there was a diagram :( I am still learning about these layers and
will need to fully understand them when we're done. This patch just makes the
implicit layer dependencies explicit.

Mind if I file the 6 params -> struct as a followup bug?

[wkorman, 2017/05/17 21:45:49]

Sure, sounds good.

+      scoped_refptr<Layer> overscroll_elasticity_layer,
+      scoped_refptr<Layer> page_scale_layer,
+      scoped_refptr<Layer> inner_viewport_container_layer,
+      scoped_refptr<Layer> outer_viewport_container_layer,
+      scoped_refptr<Layer> inner_viewport_scroll_layer,
+      scoped_refptr<Layer> outer_viewport_scroll_layer);
 
   Layer* overscroll_elasticity_layer() const {
     return overscroll_elasticity_layer_.get();
   }
   Layer* page_scale_layer() const { return page_scale_layer_.get(); }
+  Layer* inner_viewport_container_layer() const {
+    return inner_viewport_container_layer_.get();
+  }
+  Layer* outer_viewport_container_layer() const {
+    return outer_viewport_container_layer_.get();
+  }
   Layer* inner_viewport_scroll_layer() const {
     return inner_viewport_scroll_layer_.get();
   }
@@ -564,6 +573,8 @@ class CC_EXPORT LayerTreeHost : public NON_EXPORTED_BASE(SurfaceReferenceOwner),
 
   scoped_refptr<Layer> overscroll_elasticity_layer_;
   scoped_refptr<Layer> page_scale_layer_;
+  scoped_refptr<Layer> inner_viewport_container_layer_;
+  scoped_refptr<Layer> outer_viewport_container_layer_;
   scoped_refptr<Layer> inner_viewport_scroll_layer_;
   scoped_refptr<Layer> outer_viewport_scroll_layer_;