cc: Fix bug in generating picture layer pairs from 2 lists.

cc/trees/layer_tree_host_impl.cc

 // 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 1220 matching lines @@
         !twin_layer->HasValidTilePriorities()) {
       twin_layer = nullptr;
     }
     layer_pairs->push_back(PictureLayerImpl::Pair(layer, twin_layer));
   }
 
   if (pending_tree_) {
     for (auto& layer : pending_tree_->picture_layers()) {
       if (need_valid_tile_priorities && !layer->HasValidTilePriorities())
         continue;
-      if (layer->GetPendingOrActiveTwinLayer()) {
-        // Already captured from the active tree.
-        continue;
+      if (PictureLayerImpl* twin_layer = layer->GetPendingOrActiveTwinLayer()) {
+        if (!need_valid_tile_priorities ||
+            twin_layer->HasValidTilePriorities()) {
+          // Already captured from the active tree.
+          continue;
+        }
       }
       layer_pairs->push_back(PictureLayerImpl::Pair(nullptr, layer));
     }
   }
 }
 
 scoped_ptr<RasterTilePriorityQueue> LayerTreeHostImpl::BuildRasterQueue(
     TreePriority tree_priority,
     RasterTilePriorityQueue::Type type) {
   TRACE_EVENT0("cc", "LayerTreeHostImpl::BuildRasterQueue");
@@ skipping 2251 matching lines @@
     (*it)->OnSetNeedsRedrawOnImpl();
 }
 
 void LayerTreeHostImpl::NotifySwapPromiseMonitorsOfForwardingToMainThread() {
   std::set<SwapPromiseMonitor*>::iterator it = swap_promise_monitor_.begin();
   for (; it != swap_promise_monitor_.end(); it++)
     (*it)->OnForwardScrollUpdateToMainThreadOnImpl();
 }
 
 }  // namespace cc