Cleanup: Pass std::string as const reference from gpu/

gpu/tools/compositor_model_bench/render_tree.h

 // Copyright (c) 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.
 
 // Data structures for representing parts of Chromium's composited layer tree
 // and a function to load it from the JSON configuration file
 
 #ifndef GPU_TOOLS_COMPOSITOR_MODEL_BENCH_RENDER_TREE_H_
 #define GPU_TOOLS_COMPOSITOR_MODEL_BENCH_RENDER_TREE_H_
 
@@ skipping 17 matching lines @@
   int texID;
 };
 
 struct Texture {
   int texID;
   int height;
   int width;
   GLenum format;
 };
 
-GLenum TextureFormatFromString(std::string format);
+GLenum TextureFormatFromString(const std::string& format);
 const char* TextureFormatName(GLenum format);
 int FormatBytesPerPixel(GLenum format);
 
 class RenderNodeVisitor;
 
 class RenderNode {
  public:
   RenderNode();
   virtual ~RenderNode();
   virtual void Accept(RenderNodeVisitor* v);
@@ skipping 153 matching lines @@
   virtual void BeginVisitCCNode(CCNode* v);
   virtual void EndVisitRenderNode(RenderNode* v);
   virtual void EndVisitContentLayerNode(ContentLayerNode* v);
   virtual void EndVisitCCNode(CCNode* v);
 };
 
 RenderNode* BuildRenderTreeFromFile(const base::FilePath& path);
 
 #endif  // GPU_TOOLS_COMPOSITOR_MODEL_BENCH_RENDER_TREE_H_