Make sure snap_page combined iframe serialized dom

telemetry/telemetry/internal/backends/chrome_inspector/inspector_runtime.py

 # Copyright 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.
 from telemetry.core import exceptions
 
 
 class InspectorRuntime(object):
   def __init__(self, inspector_websocket):
     self._inspector_websocket = inspector_websocket
     self._inspector_websocket.RegisterDomain('Runtime', self._OnNotification)
     self._contexts_enabled = False
-    self._max_context_id = None
+    self._all_context_ids = None
 
   def _OnNotification(self, msg):
     if (self._contexts_enabled and
         msg['method'] == 'Runtime.executionContextCreated'):
-      self._max_context_id = max(self._max_context_id,
-                                 msg['params']['context']['id'])
+      self._all_context_ids.add(msg['params']['context']['id'])
 
   def Execute(self, expr, context_id, timeout):
     self.Evaluate(expr + '; 0;', context_id, timeout)
 
   def Evaluate(self, expr, context_id, timeout):
     """Evaluates a javascript expression and returns the result.
 
     |context_id| can refer to an iframe. The main page has context_id=1, the
     first iframe context_id=2, etc.
 
@@ skipping 31 matching lines @@
   def EnableAllContexts(self):
     """Allow access to iframes.
 
     Raises:
       exceptions.WebSocketDisconnected
       websocket.WebSocketException
       socket.error
     """
     if not self._contexts_enabled:
       self._contexts_enabled = True
+      self._all_context_ids = set()
       self._inspector_websocket.SyncRequest({'method': 'Runtime.enable'},
                                             timeout=30)
-    return self._max_context_id
+    return self._all_context_ids
 
   def RunInspectorCommand(self, command, timeout):
     """Runs an inspector command.
 
     Raises:
       exceptions.WebSocketDisconnected
       websocket.WebSocketException
       socket.error
     """
     res = self._inspector_websocket.SyncRequest(command, timeout)
     return res