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

cc/output/shader.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/output/shader.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/logging.h"
@@ skipping 754 matching lines @@
 
 FragmentTexBlendMode::FragmentTexBlendMode()
     : backdrop_location_(-1),
       original_backdrop_location_(-1),
       backdrop_rect_location_(-1),
       blend_mode_(BLEND_MODE_NONE),
       mask_for_background_(false) {
 }
 
 std::string FragmentTexBlendMode::SetBlendModeFunctions(
-    std::string shader_string) const {
+    const std::string& shader_string) const {
   if (shader_string.find("ApplyBlendMode") == std::string::npos)
     return shader_string;
 
   if (!has_blend_mode()) {
     return "#define ApplyBlendMode(X, Y) (X)\n" + shader_string;
   }
 
   static const std::string kUniforms = SHADER0([]() {
     uniform sampler2D s_backdropTexture;
     uniform sampler2D s_originalBackdropTexture;
@@ skipping 1461 matching lines @@
     void main() {
       vec4 d4 = min(edge_dist[0], edge_dist[1]);
       vec2 d2 = min(d4.xz, d4.yw);
       float aa = clamp(gl_FragCoord.w * min(d2.x, d2.y), 0.0, 1.0);
       gl_FragColor = color * aa;
     }
   });
 }
 
 }  // namespace cc