network_diagnostics: Update HttpFirewall routine

Update the HttpFirewall routine to use the TlsProber. This ensures the
DCHECK failure reported in crbug.com/1194907 is resolved since the
TlsProber doesn't need to be run on the IO thread.

BUG=chromium:1194907
TEST=unit_tests --gtest_filter=HttpFirewallRoutineTest*

Change-Id: Ic67716e085f0db316d15d5d8f0347471f4bfcd3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2875506
Commit-Queue: Kartik Hegde <khegde@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#884311}