In update-w3c-test-expectations, handle failure to fetch results.

third_party/WebKit/Tools/Scripts/webkitpy/common/net/buildbot_mock.py

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@@ skipping 13 matching lines @@
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 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 from webkitpy.common.net.buildbot import BuildBot
 from webkitpy.common.net.layouttestresults import LayoutTestResults
 from webkitpy.common.net.layouttestresults_unittest import LayoutTestResultsTest
 
 
+# TODO(qyearsley): Instead of canned results from another module, other unit
+# tests may be a little easier to understand if this returned None by default
+# when there are no canned results to return.
 class MockBuildBot(BuildBot):
 
     def __init__(self):
         super(MockBuildBot, self).__init__()
         # Dict of Build to canned LayoutTestResults.
         self._canned_results = {}
 
     def fetch_layout_test_results(self, _):
         return LayoutTestResults.results_from_string(LayoutTestResultsTest.example_full_results_json)
 
     def set_results(self, build, results):
         self._canned_results[build] = results
 
     def fetch_results(self, build):
         return self._canned_results.get(
             build,
             LayoutTestResults.results_from_string(LayoutTestResultsTest.example_full_results_json))