Fill out implementation of UpdateTestExpectations script

In r404402 I created the outline of the UpdateTestExpectations script that
removes lines from TestExpectations that are no longer flaky according to
results on the build waterfall. This CL actually implements the functionality
in the script and makes it basically functional.

BUG=595414

Review-Url: https://codereview.chromium.org/2125633002
Cr-Commit-Position: refs/heads/master@{#404515}
5 files changed