Upstream srvc wrkr "redirected resp" test to WPT

Reformat the test in terms of a series of distinct sub-tests with
expressive titles. This provides more expressive feedback in the event of
test failure. It also makes test results deterministic in the event of
multiple test failures.

Create a new version of this test that omits references to internal APIs
and place that in the "external" directory for eventual contribution to the
Web Platform Tests project.

Re-name the original test file to explicitly document its role as a
Chromium-specific test and limit its assertions to only those internal
details.

BUG=688116, 658249
R=falken@chromium.org

Review-Url: https://codereview.chromium.org/2878003003
Cr-Commit-Position: refs/heads/master@{#473221}
3 files changed