[pinpoint] Move _previous_builds from the Execution class to the Quest instance.

dashboard/dashboard/pinpoint/models/quest/find_isolate.py

 # Copyright 2016 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.
 
 from dashboard.pinpoint.models import isolate
 from dashboard.pinpoint.models.quest import execution
 from dashboard.pinpoint.models.quest import quest
 from dashboard.services import buildbucket_service
 
 
 BUCKET = 'master.tryserver.chromium.perf'
 
 
 class BuildError(Exception):
   """Raised when the build fails."""
 
 
 class FindIsolate(quest.Quest):
 
   def __init__(self, configuration, target):
     self._builder_name = _BuilderNameForConfiguration(configuration)
     self._target = target
 
+    self._previous_builds = {}
+
   def __eq__(self, other):
     return (isinstance(other, type(self)) and
             self._builder_name == other._builder_name)
 
   def __str__(self):
     return 'Build'
 
   def Start(self, change):
-    return _FindIsolateExecution(self._builder_name, self._target, change)
+    return _FindIsolateExecution(self._builder_name, self._target, change,
+                                 self._previous_builds)
 
 
 class _FindIsolateExecution(execution.Execution):
 
-  _previous_builds = {}
-
-  def __init__(self, builder_name, target, change):
+  def __init__(self, builder_name, target, change, previous_builds):
     super(_FindIsolateExecution, self).__init__()
     self._builder_name = builder_name
     self._target = target
     self._change = change
+    # previous_builds is shared among all Executions of the same Quest.
+    self._previous_builds = previous_builds
+
     self._build = None
 
   def _AsDict(self):
     return {
         'build': self._build,
     }
 
   def _Poll(self):
     if self._CheckCompleted():
       return
@@ skipping 101 matching lines @@
     # https://github.com/catapult-project/catapult/issues/3599
     parameters['properties'].update({
         'patch_storage': 'rietveld',
         'rietveld': change.patch.server,
         'issue': change.patch.issue,
         'patchset': change.patch.patchset,
     })
 
   # TODO: Look up Buildbucket bucket from builder_name.
   return buildbucket_service.Put(BUCKET, parameters)