Add __contains__ implementation to avoid a race in __get_item__.

__get_item__ enumerates ExtensionBackendList, and when it discovers
a matching index, it access ExtensionBackendList at that index. However,
ExtensionBackendList.__get_item__ calls Update, which can invalidate index.

BUG=411612
TEST=extension_unittest

Review URL: https://codereview.chromium.org/555823002

Cr-Commit-Position: refs/heads/master@{#293944}

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

@@ -25,6 +25,11 @@ 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):