Implement hash function prioritization for SRI.

Source/core/frame/SubresourceIntegrity.h

 // Copyright 2014 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 SubresourceIntegrity_h
 #define SubresourceIntegrity_h
 
 #include "core/CoreExport.h"
 #include "platform/Crypto.h"
 
@@ skipping 16 matching lines @@
     };
 
     static bool CheckSubresourceIntegrity(const Element&, const WTF::String& content, const KURL& resourceUrl, const Resource&);
 
 private:
     // FIXME: After the merge with the Chromium repo, this should be refactored
     // to use FRIEND_TEST in base/gtest_prod_util.h.
     friend class SubresourceIntegrityTest;
     friend class SubresourceIntegrityTest_Parsing_Test;
     friend class SubresourceIntegrityTest_ParseAlgorithm_Test;
+    friend class SubresourceIntegrityTest_Prioritization_Test;
 
     enum AlgorithmParseResult {
         AlgorithmValid,
         AlgorithmUnparsable,
         AlgorithmUnknown
     };
 
+    enum PrioritizationResult {
+        SameStrength,
+        FirstAlgorithmIsStronger,
+        SecondAlgorithmIsStronger
+    };
+
     struct IntegrityMetadata {
         WTF::String digest;
         HashAlgorithm algorithm;
     };
 
+    static PrioritizationResult getPrioritizedHashFunction(HashAlgorithm, HashAlgorithm);

[Mike West, 2015/06/03 09:13:33]

Why not return the stronger `HashAlgorithm`? The "first" and "second" thing is
strange.

[jww, 2015/06/03 19:26:56]

See other comments for details, but I've changed it to do this. The reason I
wrote it this way was to be more general in case a day comes when we have
different algorithms but are the same strength. Since that's not today's world,
though, we won't do that.

     static AlgorithmParseResult parseAlgorithm(const UChar*& begin, const UChar* end, HashAlgorithm&);
     static bool parseDigest(const UChar*& begin, const UChar* end, String& digest);
 
     static IntegrityParseResult parseIntegrityAttribute(const WTF::String& attribute, WTF::Vector<IntegrityMetadata>& metadataList, Document&);
 };
 
 } // namespace blink
 
 #endif