Add a comma delimited syntax parser and use it for Accept-CH

Source/platform/CommaDelimitedParser.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CommaDelimitedParser_h
+#define CommaDelimitedParser_h
+
+#include "platform/PlatformExport.h"
+#include "wtf/HashMap.h"
+#include "wtf/text/StringHash.h"
+
+namespace blink {
+
+class CommaDelimitedParser {

[Mike West, 2015/02/27 08:53:41]

I'm not sure this is a good name. It does parse a string, but you're treating it
like a Set, not like a parser.

How about 'CommaDelimitedHeaderSet'?

Really, this isn't much more than a typedef around a HashSet, and a parsing
function. It might make more sense to implement it that way as part of
`platform/network/HTTPParsers.h`. Something like:

```
using CommaDelimitedHeaderSet = HashSet<String, CaseFoldingHash>;
PLATFORM_EXPORT bool parseCommaDelimitedHeaderValue(const String& header,
CommaDelimitedHeaderSet& result);
```

WDYT?

+public:
+    PLATFORM_EXPORT CommaDelimitedParser(String);
+
+    PLATFORM_EXPORT bool contains(String);
+
+private:
+    HashMap<String, bool, CaseFoldingHash> m_map;

[Mike West, 2015/02/27 08:53:41]

I'd suggest a HashSet rather than a map containing a bool. :)

+};
+
+} // namespace blink
+#endif
+