Make SkSHA1 and SkM5 use common SkDigestHash result type

bench/ChecksumBench.cpp

Index: bench/ChecksumBench.cpp
===================================================================
--- bench/ChecksumBench.cpp	(revision 8685)
+++ bench/ChecksumBench.cpp	(working copy)
@@ -63,7 +63,7 @@
                 for (int i = 0; i < N; i++) {
                     SkMD5 md5;
                     md5.update(reinterpret_cast<uint8_t*>(fData), sizeof(fData));
-                    SkMD5::Digest digest;
+                    SkHashDigest digest;
                     md5.finish(digest);
                 }
             } break;
@@ -71,7 +71,7 @@
                 for (int i = 0; i < N; i++) {
                     SkSHA1 sha1;
                     sha1.update(reinterpret_cast<uint8_t*>(fData), sizeof(fData));
-                    SkSHA1::Digest digest;
+                    SkHashDigest digest;
                     sha1.finish(digest);
                 }
             } break;