Remove scroll_clip_layer dependency from viewport layer tracking

cc/trees/layer_tree_host.h

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_H_
 #define CC_TREES_LAYER_TREE_HOST_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 223 matching lines @@
   void SetNextCommitWaitsForActivation();
 
   // The LayerTreeHost tracks whether the content is suitable for Gpu raster.
   // Calling this will reset it back to not suitable state.
   void ResetGpuRasterizationTracking();
 
   void SetRootLayer(scoped_refptr<Layer> root_layer);
   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();
   }
   Layer* outer_viewport_scroll_layer() const {
     return outer_viewport_scroll_layer_.get();
   }
 
   void RegisterSelection(const LayerSelection& selection);
   const LayerSelection& selection() const { return selection_; }
 
@@ skipping 294 matching lines @@
 
   TaskGraphRunner* task_graph_runner_;
 
   SurfaceSequenceGenerator surface_sequence_generator_;
   uint32_t num_consecutive_frames_suitable_for_gpu_ = 0;
 
   scoped_refptr<Layer> root_layer_;
 
   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_;
 
   float top_controls_height_ = 0.f;
   float top_controls_shown_ratio_ = 0.f;
   bool browser_controls_shrink_blink_size_ = false;
 
   float bottom_controls_height_ = 0.f;
 
   float device_scale_factor_ = 1.f;
@@ skipping 48 matching lines @@
 
   std::vector<std::pair<sk_sp<const SkImage>, base::Callback<void(bool)>>>
       queued_image_decodes_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHost);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_H_