Make SkSHA1 and SkM5 use common SkDigestHash result type

gm/tests/outputs/compared-against-different-pixels-json/output-expected/json-summary.txt

Index: gm/tests/outputs/compared-against-different-pixels-json/output-expected/json-summary.txt
===================================================================
--- gm/tests/outputs/compared-against-different-pixels-json/output-expected/json-summary.txt	(revision 8826)
+++ gm/tests/outputs/compared-against-different-pixels-json/output-expected/json-summary.txt	(working copy)
@@ -2,10 +2,10 @@
    "actual-results" : {
       "failed" : {
          "565/selftest1" : {
-            "checksum" : 9512553915271796906
+            "checksum" : "320b5a7f0e7b69b35780e41f1b3f1937"
          },
          "8888/selftest1" : {
-            "checksum" : 14022967492765711532
+            "checksum" : "aa9f614e6cd7c81031f7c2aafad06fdc"
          }
       },
       "failure-ignored" : null,
@@ -14,11 +14,11 @@
    },
    "expected-results" : {
       "565/selftest1" : {
-         "checksums" : [ 11071285354315388429 ],
+         "checksums" : [ "73a2d3976df8027b6610ba431e89a7e0" ],
          "ignore-failure" : false
       },
       "8888/selftest1" : {
-         "checksums" : [ 16527650414256125612 ],
+         "checksums" : [ "d579b0446fcbacba94a2ef2dd48959f9" ],
          "ignore-failure" : false
       }
    }