vm_run_skia_try/write_json_summary: run skpdiff within imagediffdb, not on its own

compute_engine_scripts/telemetry/telemetry_slave_scripts/test_data/summary.json

 {
     "slave1": {
         "failedFiles": [
             {
                 "fileName": "file_3.png", 
                 "maxDiffPerChannel": 4, 
                 "numPixelsDiffering": 1, 
                 "percentPixelsDiffering": 2, 
-                "perceptualSimilarity": 0.3, 
+                "perceptualSimilarity": 5, 

[epoger, 2014/03/03 21:51:05]

Now that perceptualSimilarity is being provided by imagediffdb_mock, it has a
consistent value like the other fields.

[rmistry, 2014/03/04 13:14:19]

Acknowledged. (Rietveld needs an 'Ack' button, I should add it).

[epoger, 2014/03/04 14:44:42]

Ack.

Ack-tually, I guess it would also work for us to use the abbreviation "ack"
along with SGTM, etc.  Just clicking a button is faster, but a short word ain't
too bad.

                 "skpLocation": "gs://dummy-bucket/skps/file_.skp", 
                 "weightedDiffMeasure": 3
             }, 
             {
                 "fileName": "file_.png", 
                 "maxDiffPerChannel": 4, 
                 "numPixelsDiffering": 1, 
                 "percentPixelsDiffering": 2, 
-                "perceptualSimilarity": 0.1, 
+                "perceptualSimilarity": 5, 
                 "skpLocation": "gs://dummy-bucket/skps/file_.skp", 
                 "weightedDiffMeasure": 3
             }
         ], 
         "failedFilesCount": 2, 
         "filesLocationDiffs": "gs://dummy-bucket/output-dir/slave1/diffs", 
         "filesLocationNopatch": "gs://dummy-bucket/output-dir/slave1/nopatch-images", 
         "filesLocationWhiteDiffs": "gs://dummy-bucket/output-dir/slave1/whitediffs", 
         "filesLocationWithpatch": "gs://dummy-bucket/output-dir/slave1/withpatch-images", 
         "skpsLocation": "gs://dummy-bucket/skps"
     }
 }