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This change adds a non-hermetic manual browser test to verify
Chrome's DoH implementation against a handful of external
DoH servers. The test loops through each server, and
attempts to load a test URL. Fallback to Do53 is
disabled, so a successful navigation indicates that DoH passed.

A new setter set_allow_network_access_to_host_resolutions has
been added to BrowserTestBase to allow browser tests to opt-in
to using the full host resolver stack (instead of the mock host
resolver). This mode then invokes
SetAllowNetworkAccessToHostResolutions() on the NetworkServiceTest
interface to disable the test host resolver.

Bug: 1032710
Change-Id: I8e9e029e69a9e8281adb23e55179d9dd94707ef5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1961006
Commit-Queue: Kaustubha Govind <kaustubhag@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727778}