Do not instantiate base::FieldTrialList explicitly in tests

The test suite already instantiates a FieldTrialList as per
https://chromium-review.googlesource.com/c/chromium/src/+/1883567 so
it's no longer necessary to do so explicitly in tests.

This patch addresses unit tests under:
/services/network

This CL was uploaded by git cl split.

R=yhirano@chromium.org

Bug: 1018667
Change-Id: I8f004aba1ca8d194fd535080152a863f3617ddab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917233
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715735}

services/network/network_context_unittest.cc

@@ -853,7 +853,6 @@ TEST_F(NetworkContextTest, DiskCache) {
 // This makes sure that network_session_configurator::ChooseCacheType is
 // connected to NetworkContext.
 TEST_F(NetworkContextTest, SimpleCache) {
-  base::FieldTrialList field_trials(nullptr);
   base::FieldTrialList::CreateFieldTrial("SimpleCacheTrial", "ExperimentYes");
   mojom::NetworkContextParamsPtr context_params = CreateContextParams();
   context_params->http_cache_enabled = true;