cc: Implement geometry-based tile eviction

cc/resources/eviction_tile_priority_queue.cc

 // Copyright 2014 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.
 
 #include "cc/resources/eviction_tile_priority_queue.h"
 
 namespace cc {
 
 namespace {
 
@@ skipping 190 matching lines @@
   const Tile* active_tile = active_queue->Top();
   const Tile* pending_tile = pending_queue->Top();
   if (active_tile == pending_tile)
     return ACTIVE_TREE;
 
   const TilePriority& active_priority =
       active_tile->priority_for_tree_priority(tree_priority);
   const TilePriority& pending_priority =
       pending_tile->priority_for_tree_priority(tree_priority);
 
+  if (active_priority.priority_bin == pending_priority.priority_bin) {

[vmpstr, 2014/12/09 02:10:58]

While here, can you add a couple more comments in this functions:

at line 202, something like:
// If tiles are the same, it doesn't matter which tree we return.

before this block, something like:
// If the bins are the same and activation differs, then return the tree of the
tile not required for activaiton.

and below this block:
// Return tile with a lower priority.

[USE eero AT chromium.org, 2014/12/09 18:44:14]

Done.

+    if (active_tile->required_for_activation()) {
+      if (!pending_tile->required_for_activation())

[vmpstr, 2014/12/09 02:10:58]

I think this can be a bit simpler:

if (active_priority.priority_bin == pending_priority.priority_bin &&
    active_tile->required_for_activation() !=    
        pending_tile_required_for_activation()) {
 return active_tile->required_for_activation()
            ? PENDING_TREE : ACTIVE_TREE;
}

That works, right?

[USE eero AT chromium.org, 2014/12/09 18:44:14]

Yes. Done.

+        return PENDING_TREE;
+    } else {
+      if (pending_tile->required_for_activation())
+        return ACTIVE_TREE;
+    }
+  }
+
   if (pending_priority.IsHigherPriorityThan(active_priority))
     return ACTIVE_TREE;
   return PENDING_TREE;
 }
 
 }  // namespace cc