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:
/content/renderer

This CL was uploaded by git cl split.

R=szager@chromium.org

Bug: 1018667
Change-Id: Ib9a7ef23d1a670d9060c8a3aae5bfa13dae329a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917365
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717310}

content/renderer/render_thread_impl_browsertest.cc

@@ -154,7 +154,7 @@ class QuitOnTestMsgFilter : public IPC::MessageFilter {
 
 class RenderThreadImplBrowserTest : public testing::Test {
  public:
-  RenderThreadImplBrowserTest() : field_trial_list_(nullptr) {}
+  RenderThreadImplBrowserTest() {}
 
   void SetUp() override {
     content_renderer_client_.reset(new ContentRendererClient());
@@ -281,8 +281,6 @@ class RenderThreadImplBrowserTest : public testing::Test {
   // get killed by the OS. Memory leaks aren't nice but it's test-only.
   RenderThreadImpl* thread_;
 
-  base::FieldTrialList field_trial_list_;
-
   std::unique_ptr<base::RunLoop> run_loop_;
 
  private: