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

cc/output/shader.cc

Index: cc/output/shader.cc
diff --git a/cc/output/shader.cc b/cc/output/shader.cc
index 4a877ae859f6e6da9bcd4d759881fdfe78e45f00..a4ee1bbfc6baa38661c4b2cfb3e08800a880c653 100644
--- a/cc/output/shader.cc
+++ b/cc/output/shader.cc
@@ -772,7 +772,7 @@ FragmentTexBlendMode::FragmentTexBlendMode()
 }
 
 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;