Implement hash function prioritization for SRI.

Source/core/frame/SubresourceIntegrity.h

Index: Source/core/frame/SubresourceIntegrity.h
diff --git a/Source/core/frame/SubresourceIntegrity.h b/Source/core/frame/SubresourceIntegrity.h
index 6fdf17f710bd508b859e721140a8709bc3d6e6f6..d81363ec729f37202f01586963c3188bd2926f1d 100644
--- a/Source/core/frame/SubresourceIntegrity.h
+++ b/Source/core/frame/SubresourceIntegrity.h
@@ -34,6 +34,7 @@ private:
     friend class SubresourceIntegrityTest;
     friend class SubresourceIntegrityTest_Parsing_Test;
     friend class SubresourceIntegrityTest_ParseAlgorithm_Test;
+    friend class SubresourceIntegrityTest_Prioritization_Test;
 
     enum AlgorithmParseResult {
         AlgorithmValid,
@@ -41,11 +42,18 @@ private:
         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);