Clean up URLFetcher unit tests, part 8.
Clean up URLFetcher unit tests, part 8.

Convert the CancelTests to use the shared test fixture, and add a
CancelTest for sync cancellation.  Remove a bunch of dead code.
Make fetcher tests where the network stack and fetcher are on different
threads easier to set up.

BUG=471069

Review URL: https://codereview.chromium.org/1096653003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#328366}

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