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/ui/search

This CL was uploaded by git cl split.

R=gayane@chromium.org

Bug: 1018667
Change-Id: Icb63ab7435f32f6613a9dd06b551d6ebab960303
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917508
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: Gayane Petrosyan <gayane@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715868}

chrome/browser/ui/search/search_ipc_router_unittest.cc

@@ -170,7 +170,7 @@ class MockEmbeddedSearchClientFactory
 
 class SearchIPCRouterTest : public BrowserWithTestWindowTest {
  public:
-  SearchIPCRouterTest() : field_trial_list_(NULL) {}
+  SearchIPCRouterTest() {}
 
   void SetUp() override {
     BrowserWithTestWindowTest::SetUp();
@@ -252,7 +252,6 @@ class SearchIPCRouterTest : public BrowserWithTestWindowTest {
 
  private:
   MockSearchIPCRouterDelegate delegate_;
-  base::FieldTrialList field_trial_list_;
   MockEmbeddedSearchClient mock_embedded_search_client_;
 };