Make SkSHA1 and SkM5 use common SkDigestHash result type

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

Index: gm/tests/outputs/compared-against-different-pixels-images/output-expected/json-summary.txt
===================================================================
--- gm/tests/outputs/compared-against-different-pixels-images/output-expected/json-summary.txt	(revision 8685)
+++ gm/tests/outputs/compared-against-different-pixels-images/output-expected/json-summary.txt	(working copy)
@@ -2,10 +2,10 @@
    "actual-results" : {

[epoger, 2013/04/18 17:45:48]

Ready for renewed discussion at patchset 8.

(Patchsets 6/7/8 are all the same--"gcl upload" failed on patchsets 6 and 7, so
I just kept retrying until it worked)

[epoger, 2013/04/18 18:07:13]

I should have made this clearer: please see the diffs between patchset 5 and
patchset 8.  That set of changes just switched the hashing algorithm used "under
the covers" in SkBitmapHasher.cpp.

It it my view that the simplicity of switching algorithms justifies the
existence of the SkHashDigest class, or perhaps some different wrapper around
the hash results.  If so, we can then examine the totality of this CL and look
for cleanups...

       "failed" : {
          "565/selftest1" : {
-            "checksum" : 9512553915271796906
+            "checksum" : "320b5a7f0e7b69b35780e41f1b3f1937"

[epoger, 2013/04/18 17:45:48]

updated gm self-test results with MD5 digests instead of CityHash

          },
          "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
       }
    }