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Use matchers in SystemNetworkContextManagerBrowsertest.

Using matchers (instead of multiple, simpler expectations) is more
declarative, less repetitive, and allows the test to give better error
messages in case of failure. In this test in particular, the matchers
allow us to avoid crashing the test if `secure_dns_config.servers()`
contains the wrong number of elements; and allows us to delete the
`ContainsSubstring` helper.

Change-Id: Ia0f274bbe19a009e7cefbea43ff7fb33c31463be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488220
Commit-Queue: Chris Fredrickson <cfredric@google.com>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Auto-Submit: Chris Fredrickson <cfredric@google.com>
Cr-Commit-Position: refs/heads/master@{#820260}