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..2916848a270b3b7d7dbe15afcbf805c491d5fe2f 100644
--- a/base/strings/string_util.h
+++ b/base/strings/string_util.h
@@ -429,11 +429,28 @@ BASE_EXPORT char16* WriteInto(string16* str, size_t length_with_null);
 BASE_EXPORT wchar_t* WriteInto(std::wstring* str, size_t length_with_null);
 #endif
 
-// Does the opposite of SplitString().
+// Does the opposite of SplitString()/SplitStringPiece(). Joins a vector or list
+// of strings into a single string, inserting |separator| (which may be empty)
+// in between all elements.
+//
+// Prefer the StringPiece variant if possible, to avoid unnecessary copying.
 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 JoinString(const std::vector<StringPiece>& parts,
+                                   StringPiece separator);
+BASE_EXPORT string16 JoinString(const std::vector<StringPiece16>& parts,
+                                StringPiece16 separator);
+// Explicit initializer_list overloads are required to break ambiguity when used
+// with a literal initializer list (otherwise the compiler would not be able to
+// decide between the two other overloads).
+BASE_EXPORT std::string JoinString(
+    const std::initializer_list<StringPiece>& parts,
+    StringPiece separator);
+BASE_EXPORT string16
+JoinString(const std::initializer_list<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