Check if priority actually changed in Tile::SetPriority()

cc/resources/tile.cc

 // Copyright 2012 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/tile.h"
 
 #include "base/stringprintf.h"
 #include "cc/resources/tile_manager.h"
 #include "third_party/khronos/GLES2/gl2.h"
 
@@ skipping 28 matching lines @@
   res->SetString("picture_pile", base::StringPrintf("%p",
                                                     picture_pile_.get()));
   res->SetDouble("contents_scale", contents_scale_);
   res->Set("priority.0", priority_[ACTIVE_TREE].AsValue().release());
   res->Set("priority.1", priority_[PENDING_TREE].AsValue().release());
   res->Set("managed_state", managed_state_.AsValue().release());
   return res.PassAs<base::Value>();
 }
 
 void Tile::SetPriority(WhichTree tree, const TilePriority& priority) {
+  if (priority_[tree] == priority)
+    return;
+
   tile_manager_->WillModifyTilePriority(this, tree, priority);
   priority_[tree] = priority;
 }
 
 }  // namespace cc