string_util support for joining strings using strings not just chars.

base/string_util.h

Index: base/string_util.h
diff --git a/base/string_util.h b/base/string_util.h
index 428550baef573ec67f14ffffa9d8fc30537e958b..367eaa8e1bd4257ecbb7d03282cb370ea7d252e4 100644
--- a/base/string_util.h
+++ b/base/string_util.h
@@ -508,6 +508,14 @@ BASE_EXPORT string16 JoinString(const std::vector<string16>& parts, char16 s);
 BASE_EXPORT std::string JoinString(
     const std::vector<std::string>& parts, char s);
 
+// Join |parts| using |separator|.
+BASE_EXPORT std::string JoinString(
+    const std::vector<std::string>& parts,
+    const std::string& separator);
+BASE_EXPORT string16 JoinString(
+    const std::vector<string16>& parts,
+    const string16& separator);
+
 // Replace $1-$2-$3..$9 in the format string with |a|-|b|-|c|..|i| respectively.
 // Additionally, any number of consecutive '$' characters is replaced by that
 // number less one. Eg $$->$, $$$->$$, etc. The offsets parameter here can be