Introduce GetBackendFromContextId to avoid using list index.

BUG=398467
TEST=ExtensionTest

Review URL: https://codereview.chromium.org/556003005

Cr-Commit-Position: refs/heads/master@{#294455}

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

@@ -160,9 +160,11 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
            document.readyState == 'interactive')
       """
       for e in self._extensions_to_load:
-        if not e.extension_id in self.extension_backend:
+        try:
+          extension_objects = self.extension_backend[e.extension_id]
+        except KeyError:
           return False
-        for extension_object in self.extension_backend[e.extension_id]:
+        for extension_object in extension_objects:
           try:
             res = extension_object.EvaluateJavaScript(
                 extension_ready_js % e.extension_id)

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

@@ -25,16 +25,12 @@ class ExtensionBackendDict(collections.Mapping):
   def __init__(self, browser_backend):
     self._extension_backend_list = ExtensionBackendList(browser_backend)
 
-  def __contains__(self, extension_id):
-    return (extension_id in
-        (self.ContextIdToExtensionId(context_id)
-         for context_id in self._extension_backend_list))
-
   def __getitem__(self, extension_id):
     extensions = []
-    for i, context_id in enumerate(self._extension_backend_list):
+    for context_id in self._extension_backend_list:
       if self.ContextIdToExtensionId(context_id) == extension_id:
-        extensions.append(self._extension_backend_list[i])
+        extensions.append(
+            self._extension_backend_list.GetBackendFromContextId(context_id))
     if not extensions:
       raise KeyError('Cannot find an extension with id=%s' % extension_id)
     return extensions

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,