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This concludes the refactoring. The tests now have a MUCH clearer
control flow, and test success is no longer handled behind the scenes.

I found that it's better to keep promise chains as long as possible,
since we can then append a single .catch(failTest) at the end. This
isn't strictly necessary - the test will time out since an error
will break the promise chain and not call reportTestSuccess, but
it's much nicer if the test fails right away on exceptions, rather
than timing out after 45 seconds.

Bug: chromium:777857
Change-Id: I46c110782488bb20cdf7101690ca6966601506f2
Reviewed-on: https://chromium-review.googlesource.com/793153
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519695}