Make SkSHA1 and SkM5 use common SkDigestHash result type

src/utils/SkMD5.h

Index: src/utils/SkMD5.h
===================================================================
--- src/utils/SkMD5.h	(revision 8685)
+++ src/utils/SkMD5.h	(working copy)
@@ -8,9 +8,10 @@
 #ifndef SkMD5_DEFINED
 #define SkMD5_DEFINED
 
-#include "SkTypes.h"
 #include "SkEndian.h"
+#include "SkHashDigest.h"
 #include "SkStream.h"
+#include "SkTypes.h"
 
 //The following macros can be defined to affect the MD5 code generated.
 //SK_MD5_CLEAR_DATA causes all intermediate state to be overwritten with 0's.
@@ -32,12 +33,8 @@
     /** Processes input, adding it to the digest. Calling this after finish is undefined. */
     void update(const uint8_t* input, size_t length);
 
-    struct Digest {
-        uint8_t data[16];
-    };
-
     /** Computes and returns the digest. */
-    void finish(Digest& digest);
+    void finish(SkHashDigest& digest);
 
 private:
     // number of bytes, modulo 2^64