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:
/chrome/browser/tracing

This CL was uploaded by git cl split.

R=eseckler@chromium.org

Bug: 1018667
Change-Id: I8b22ba4e94324550545d39beaa65e65df8ee22a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916470
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715275}

chrome/browser/tracing/background_tracing_field_trial_unittest.cc

@@ -58,7 +58,6 @@ void CheckConfig(std::string* config) {
 }  // namespace
 
 TEST_F(BackgroundTracingTest, SetupBackgroundTracingFieldTrial) {
-  base::FieldTrialList field_trial_list(nullptr);
   const std::string kTrialName = "BackgroundTracing";
   const std::string kExperimentName = "SlowStart";
   base::AssociateFieldTrialParams(kTrialName, kExperimentName,