Add support for python pageset

tools/telemetry/telemetry/page/page_set.py

Index: tools/telemetry/telemetry/page/page_set.py
diff --git a/tools/telemetry/telemetry/page/page_set.py b/tools/telemetry/telemetry/page/page_set.py
index f97cb0d69cf900979e158d5dc2fcf0105a6dad92..0841d9b0c7ef90befbeff4b549b2c4254cb231d6 100644
--- a/tools/telemetry/telemetry/page/page_set.py
+++ b/tools/telemetry/telemetry/page/page_set.py
@@ -86,6 +86,7 @@ class PageSet(object):
 
   @classmethod
   def FromFile(cls, file_path):
+    assert file_path.endswith('.json'), 'This pageset only supports json file'
     with open(file_path, 'r') as f:
       contents = f.read()
     data = json.loads(contents)
@@ -96,18 +97,16 @@ class PageSet(object):
     return cls(file_path, data)
 
   @property
+  def is_python_pageset(self):
+    return False
+
+  @property
   def _base_dir(self):
     if os.path.isfile(self.file_path):
       return os.path.dirname(self.file_path)
     else:
       return self.file_path
 
-  def ContainsOnlyFileURLs(self):
-    for page in self.pages:
-      if not page.is_file:
-        return False
-    return True
-
   def ReorderPageSet(self, results_file):
     """Reorders this page set based on the results of a past run."""
     page_set_dict = {}