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This is more preparation for changing EmbeddedWorkerTestHelper tests
to use Mojo.

The job tests were overly constrained about what tasks ran when, when
they just need to make assertions about whether a version became
activated, etc. This introduces a mechanism to wait for a version status
change, instead of relying on various RunUntilIdle()-like calls.

More details: My plan is to change the RequestTermination() mechanism in
this file to use the Mojo method
EmbeddedWorkerInstanceHost::RequestTermination() rather than calling
ServiceWorkerVersion::RequestTermination() directly. But in the Mojo
version, it's not easy to just wait for the callback, since the browser
actually sends back StopWorker() first and then the callback. The
default mock client would immediately simulate stopping and sever
the Mojo connection in StopWorker(), so the RequestTermination callback
is never invoked.

But these tests shouldn't be concerned with the details of
RequestTermination() callback and StopWorker() order. They just want to
assert that an update occurred.

Bug: 927651
Change-Id: I0b4729f731e1880f0731d7ba4f766b8b52e8b93c
Reviewed-on: https://chromium-review.googlesource.com/c/1454203
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629619}