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

compute_engine_scripts/telemetry/telemetry_slave_scripts/test_data/imagediffdb_mock.py

-#!/usr/bin/env python                                                           
-# Copyright (c) 2013 The Chromium Authors. All rights reserved.                 
-# Use of this source code is governed by a BSD-style license that can be        
+#!/usr/bin/env python
+# Copyright (c) 2013 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Dummy module that pretends to be imagediffdb.py for write_json_summary_test.
 
 imagediffdb.py here refers to
 https://code.google.com/p/skia/source/browse/trunk/gm/rebaseline_server/imagediffdb.py
+
+TODO(rmistry): As noted in https://codereview.chromium.org/183763025 ,
+it would be good for us to add lots of assertions about how this mock gets
+called during unittests.
 """
 
 
 class DiffRecord(object):
 
-  def __init__(self, storage_root, expected_image_url, expected_image_locator,
-               actual_image_url, actual_image_locator, expected_images_subdir,
-               actual_images_subdir):
+  def __init__(self, storage_root=None, expected_image_url=None,
+               expected_image_locator=None, actual_image_url=None,
+               actual_image_locator=None, expected_images_subdir=None,
+               actual_images_subdir=None):
     pass
 
   def get_num_pixels_differing(self):
     return 1
 
   def get_percent_pixels_differing(self):
     return 2
 
   def get_weighted_diff_measure(self):
     return 3
 
   def get_max_diff_per_channel(self):
     return 4
 
+  def get_perceptual_difference(self):
+    return 5
+
+
+class ImageDiffDB(object):
+
+  def __init__(self, storage_root):
+    pass
+
+  def add_image_pair(self, expected_image_url, expected_image_locator,
+                     actual_image_url, actual_image_locator):
+    pass
+
+  def get_diff_record(self, expected_image_locator, actual_image_locator):
+    return DiffRecord()