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:
/components/subresource_filter

This CL was uploaded by git cl split.

R=csharrison@chromium.org

Bug: 1018667
Change-Id: I72995e8d6b1ec8eccdab2fd5653419eb2953de64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916471
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715302}

components/subresource_filter/content/browser/subresource_filter_safe_browsing_activation_throttle_unittest.cc

@@ -13,7 +13,6 @@
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
-#include "base/metrics/field_trial.h"
 #include "base/run_loop.h"
 #include "base/test/metrics/histogram_tester.h"
 #include "base/test/scoped_feature_list.h"
@@ -141,8 +140,8 @@ class SubresourceFilterSafeBrowsingActivationThrottleTest
     : public content::RenderViewHostTestHarness,
       public content::WebContentsObserver {
  public:
-  SubresourceFilterSafeBrowsingActivationThrottleTest()
-      : field_trial_list_(nullptr) {}
+  SubresourceFilterSafeBrowsingActivationThrottleTest() {}
+
   ~SubresourceFilterSafeBrowsingActivationThrottleTest() override {}
 
   void SetUp() override {
@@ -330,7 +329,6 @@ class SubresourceFilterSafeBrowsingActivationThrottleTest
   }
 
  private:
-  base::FieldTrialList field_trial_list_;
   testing::ScopedSubresourceFilterConfigurator scoped_configuration_;
   scoped_refptr<base::TestMockTimeTaskRunner> test_io_task_runner_;