Remove legacy API tests for HostResolver.

Also:
*Update deprecation comments on legacy API methods now that they are
 untested.
*Added a new-API test for changing priority. Support was added a while
 back, but I must've forgotten to add a test.
*Some minor TODO cleanups for tests that already exist.

Will not commit this until confirming all non-test usage of the legacy
API is gone (should just be waiting for crrev.com/c/1446780 to land).

Bug: 922699
Change-Id: Ia5afe66883baaf3b321f3195e1fd4ff109e00ddf
Reviewed-on: https://chromium-review.googlesource.com/c/1479750
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634455}
2 files changed