Make SkSHA1 and SkM5 use common SkDigestHash result type

src/utils/SkSHA1.h

Index: src/utils/SkSHA1.h
===================================================================
--- src/utils/SkSHA1.h	(revision 8685)
+++ src/utils/SkSHA1.h	(working copy)
@@ -8,6 +8,7 @@
 #ifndef SkSHA1_DEFINED
 #define SkSHA1_DEFINED
 
+#include "SkHashDigest.h"
 #include "SkTypes.h"
 #include "SkEndian.h"
 #include "SkStream.h"
@@ -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[20];
-    };
-
     /** Computes and returns the digest. */
-    void finish(Digest& digest);
+    void finish(SkHashDigest& digest);
 
 private:
     // number of bytes, modulo 2^64