Make credential_path a page attribute instead of page_set attribute.

tools/telemetry/telemetry/page/__init__.py

Index: tools/telemetry/telemetry/page/__init__.py
diff --git a/tools/telemetry/telemetry/page/__init__.py b/tools/telemetry/telemetry/page/__init__.py
index 2435080a8460799d523cf35cc7ce8ecfade86dd5..4352e41ba0d82cda6e577333f0579fe75d77d97f 100644
--- a/tools/telemetry/telemetry/page/__init__.py
+++ b/tools/telemetry/telemetry/page/__init__.py
@@ -2,15 +2,28 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 import inspect
+import logging
 import os
 import urlparse
 
 from telemetry import user_story
+from telemetry.util import cloud_storage
+from telemetry.util import path
 
 
-class Page(user_story.UserStory):
+def _UpdateCredentials(credentials_path):
+  # Attempt to download the credentials file.
+  try:
+    cloud_storage.GetIfChanged(credentials_path)
+  except (cloud_storage.CredentialsError, cloud_storage.PermissionError,
+          cloud_storage.CloudStorageError) as e:
+    logging.warning('Cannot retrieve credential file %s due to cloud storage '
+                    'error %s', credentials_path, str(e))
+
 
-  def __init__(self, url, page_set=None, base_dir=None, name=''):
+class Page(user_story.UserStory):
+  def __init__(self, url, page_set=None, base_dir=None, name='',
+               credentials_path=None):
     super(Page, self).__init__(name)
     self._url = url
     self._page_set = page_set
@@ -20,7 +33,13 @@ class Page(user_story.UserStory):
       base_dir = os.path.dirname(inspect.getfile(self.__class__))
     self._base_dir = base_dir
     self._name = name
-
+    if credentials_path:
+      credentials_path = os.path.join(self._base_dir, credentials_path)
+      _UpdateCredentials(credentials_path)
+      if not os.path.exists(credentials_path):
+        logging.error('Invalid credentials path: %s' % credentials_path)
+        credentials_path = None
+    self._credentials_path = credentials_path
 
     # These attributes can be set dynamically by the page.
     self.synthetic_delays = dict()
@@ -30,6 +49,10 @@ class Page(user_story.UserStory):
     self.script_to_evaluate_on_commit = None
     self._SchemeErrorCheck()
 
+  @property
+  def credentials_path(self):
+    return self._credentials_path
+
   def _SchemeErrorCheck(self):
     if not self._scheme:
       raise ValueError('Must prepend the URL with scheme (e.g. file://)')
@@ -141,6 +164,10 @@ class Page(user_story.UserStory):
         self._base_dir, parsed_url.netloc + parsed_url.path))
 
   @property
+  def base_dir(self):
+    return self._base_dir
+
+  @property
   def file_path_url(self):
     """Returns the file path, including the params, query, and fragment."""
     assert self.is_file