Use StringView for String::split.

third_party/WebKit/Source/wtf/text/WTFString.h

 /*
  * (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013 Apple Inc.
  * All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 297 matching lines @@
   // Returns an uninitialized string. The characters needs to be written
   // into the buffer returned in data before the returned string is used.
   // Failure to do this will have unpredictable results.
   static String createUninitialized(unsigned length, UChar*& data) {
     return StringImpl::createUninitialized(length, data);
   }
   static String createUninitialized(unsigned length, LChar*& data) {
     return StringImpl::createUninitialized(length, data);
   }
 
-  void split(const String& separator,
+  void split(const StringView& separator,
              bool allowEmptyEntries,
              Vector<String>& result) const;
-  void split(const String& separator, Vector<String>& result) const {
+  void split(const StringView& separator, Vector<String>& result) const {
     split(separator, false, result);
   }
   void split(UChar separator,
              bool allowEmptyEntries,
              Vector<String>& result) const;
   void split(UChar separator, Vector<String>& result) const {
     split(separator, false, result);
   }
 
   // Copy characters out of the string. See StringImpl.h for detailed docs.
@@ skipping 298 matching lines @@
 using WTF::String;
 using WTF::emptyString;
 using WTF::emptyString16Bit;
 using WTF::charactersAreAllASCII;
 using WTF::equal;
 using WTF::find;
 using WTF::isSpaceOrNewline;
 
 #include "wtf/text/AtomicString.h"
 #endif  // WTFString_h