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:
/chromecast

This CL was uploaded by git cl split.

R=yucliu@chromium.org

Bug: 1018667
Change-Id: I5a8328c5e68585b67e8bc0c64acb4182b4b2ef45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916469
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715360}

chromecast/base/cast_features_unittest.cc

@@ -5,7 +5,6 @@
 #include "chromecast/base/cast_features.h"
 
 #include "base/macros.h"
-#include "base/metrics/field_trial.h"
 #include "base/metrics/field_trial_params.h"
 #include "base/values.h"
 #include "testing/gmock/include/gmock/gmock.h"
@@ -25,7 +24,7 @@ const char kTestParamsFeatureName[] = "test_params_feature";
 
 class CastFeaturesTest : public testing::Test {
  public:
-  CastFeaturesTest() : field_trial_list_(nullptr) {}
+  CastFeaturesTest() {}
   ~CastFeaturesTest() override {}
 
   // testing::Test implementation:
@@ -40,10 +39,6 @@ class CastFeaturesTest : public testing::Test {
   }
 
  private:
-  // A field trial list must be created before attempting to create FieldTrials.
-  // In production, this instance lives in CastBrowserMainParts.
-  base::FieldTrialList field_trial_list_;
-
   std::unique_ptr<base::FeatureList> original_feature_list_;
 
   DISALLOW_COPY_AND_ASSIGN(CastFeaturesTest);