GM: now that we use MD5 instead of CityHash, the JSON should say so

gm/gm_expectations.h

Index: gm/gm_expectations.h
===================================================================
--- gm/gm_expectations.h	(revision 8994)
+++ gm/gm_expectations.h	(working copy)
@@ -101,7 +101,7 @@
         /**
          * Returns true iff there are no allowed checksums.
          */
-        bool empty() const { return this->fAllowedBitmapCityhashes.empty(); }
+        bool empty() const { return this->fAllowedBitmapChecksums.empty(); }
 
         /**
          * Returns true iff actualChecksum matches any allowedChecksum,
@@ -128,7 +128,7 @@
     private:
         const static bool kDefaultIgnoreFailure = false;
 
-        SkTArray<Checksum> fAllowedBitmapCityhashes;
+        SkTArray<Checksum> fAllowedBitmapChecksums;
         bool fIgnoreFailure;
         SkBitmap fBitmap;
     };