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This test was using mock.py's assert_not_called method but that does
not exist in the version in third_party/pymock. The result is that the
assert call was mocked out, allowing the test to pass without actually
testing anything.

Fix is two-fold:
1 factor out the filtering code and test it directly
2 assert that .call_count is 0

Bug: 890657
Change-Id: Iffc6382ad0d9335a11c17f5ffe4d0c6ae6cbcf6c
Reviewed-on: https://chromium-review.googlesource.com/1253601
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595406}