Added StringPiece overloads for base::JoinString.

base/strings/string_util.h

Index: base/strings/string_util.h
diff --git a/base/strings/string_util.h b/base/strings/string_util.h
index e5cce8fd97e56f93e0717250b2b4bb60851c4806..5d3bdf9ae66dc3a19ce6cf9b561cc797401c1af9 100644
--- a/base/strings/string_util.h
+++ b/base/strings/string_util.h
@@ -434,6 +434,10 @@ BASE_EXPORT std::string JoinString(const std::vector<std::string>& parts,
                                    StringPiece separator);
 BASE_EXPORT string16 JoinString(const std::vector<string16>& parts,
                                 StringPiece16 separator);
+BASE_EXPORT std::string JoinStringPiece(const std::vector<StringPiece>& parts,
+                                        StringPiece separator);
+BASE_EXPORT string16 JoinStringPiece(const std::vector<StringPiece16>& parts,
+                                     StringPiece16 separator);
 
 // Replace $1-$2-$3..$9 in the format string with values from |subst|.
 // Additionally, any number of consecutive '$' characters is replaced by that