Cleanup: Initialize Value from tools/gn by moving std::string

tools/gn/value.h

Index: tools/gn/value.h
diff --git a/tools/gn/value.h b/tools/gn/value.h
index 44fba4adb2af16a450eda4608a767ed178a2bb1e..edcc2486cc35e7b0a155cdd93d071bf1a3650562 100644
--- a/tools/gn/value.h
+++ b/tools/gn/value.h
@@ -32,7 +32,7 @@ class Value {
   Value(const ParseNode* origin, Type t);
   Value(const ParseNode* origin, bool bool_val);
   Value(const ParseNode* origin, int64_t int_val);
-  Value(const ParseNode* origin, std::string str_val);
+  Value(const ParseNode* origin, const std::string& str_val);

[brettw, 2015/09/24 20:15:31]

I think without adding the && constructor, this will be a bad change since it
will prevent certain types of optimizations. I suspect we should just revert the
change to this file.

[ki.stfu, 2015/09/24 20:54:29]

Probably yes. BTW, why we don't want to use a movable version? You said it's
disallowed, but actually it's not. It just should be approved by OWNERS of
src/styleguide/c++/.

   Value(const ParseNode* origin, const char* str_val);
   // Values "shouldn't" have null scopes when type == Scope, so be sure to
   // always set one. However, this is not asserted since there are some