Cleanup: Pass std::string as const reference if possible

cc/output/shader.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_OUTPUT_SHADER_H_
 #define CC_OUTPUT_SHADER_H_
 
 #include <string>
 
 #include "base/basictypes.h"
@@ skipping 353 matching lines @@
   void set_blend_mode(BlendMode blend_mode) { blend_mode_ = blend_mode; }
   bool has_blend_mode() const { return blend_mode_ != BLEND_MODE_NONE; }
   void set_mask_for_background(bool mask_for_background) {
     mask_for_background_ = mask_for_background;
   }
   bool mask_for_background() const { return mask_for_background_; }
 
  protected:
   FragmentTexBlendMode();
 
-  std::string SetBlendModeFunctions(std::string shader_string) const;
+  std::string SetBlendModeFunctions(const std::string& shader_string) const;
 
   int backdrop_location_;
   int original_backdrop_location_;
   int backdrop_rect_location_;
 
  private:
   BlendMode blend_mode_;
   bool mask_for_background_;
 
   std::string GetHelperFunctions() const;
@@ skipping 479 matching lines @@
 
  private:
   int color_location_;
 
   DISALLOW_COPY_AND_ASSIGN(FragmentShaderColorAA);
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_SHADER_H_