[pinpoint] Change refactor.

dashboard/dashboard/pinpoint/models/change/patch.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.
+
+import collections
+
+
+class Patch(collections.namedtuple('Patch', ('server', 'issue', 'patchset'))):
+  """A patch in Rietveld."""
+  # TODO: Support Gerrit.
+  # https://github.com/catapult-project/catapult/issues/3599
+
+  def __str__(self):
+    return self.id_string
+
+  @property
+  def id_string(self):
+    return '%s/%d/%d' % (self.server, self.issue, self.patchset)
+
+  def AsDict(self):
+    return self._asdict()
+
+  @classmethod
+  def FromDict(cls, data):
+    # TODO: Validate to ensure the patch exists on the server.
+    return cls(data['server'], data['issue'], data['patchset'])