Introduce GetBackendFromContextId to avoid using list index.

tools/telemetry/telemetry/core/backends/chrome/inspector_backend_list.py

Index: tools/telemetry/telemetry/core/backends/chrome/inspector_backend_list.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/inspector_backend_list.py b/tools/telemetry/telemetry/core/backends/chrome/inspector_backend_list.py
index 2f7385ee197d89bb8cb0692afaa8472b00933932..66547b005fef3cc3aec2adbab8244a996d1ad1c1 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/inspector_backend_list.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/inspector_backend_list.py
@@ -42,6 +42,8 @@ class InspectorBackendList(collections.Sequence):
     """Override this method to control which contexts are included."""
     return True
 
+  #TODO(nednguyen): Remove this method and turn inspector_backend_list API to
+  # dictionary-like API (crbug.com/398467)
   def __getitem__(self, index):
     self._Update()
     if index >= len(self._inspectable_contexts_dict.keys()):
@@ -51,6 +53,12 @@ class InspectorBackendList(collections.Sequence):
                       "keys": self._inspectable_contexts_dict.keys()
                     }))
     context_id = self._inspectable_contexts_dict.keys()[index]
+    return self.GetBackendFromContextId(context_id)
+
+  def GetBackendFromContextId(self, context_id):
+    self._Update()
+    if context_id not in self._inspectable_contexts_dict:
+      raise KeyError('Cannot find a context with id=%s' % context_id)
     if context_id not in self._inspector_backend_dict:
       backend = inspector_backend.InspectorBackend(
           self._browser_backend,