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/public/test

This CL was uploaded by git cl split.

R=sky@chromium.org

Bug: 1018667
Change-Id: I71fc1531d779cc0d4d1b1cc7255b5a53f445e6eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916867
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715324}

content/public/test/render_view_test.h

@@ -25,10 +25,6 @@
 #include "third_party/blink/public/platform/platform.h"
 #include "third_party/blink/public/web/web_frame.h"
 
-namespace base {
-class FieldTrialList;
-}
-
 namespace blink {
 namespace scheduler {
 class WebThreadScheduler;
@@ -221,7 +217,6 @@ class RenderViewTest : public testing::Test {
   std::unique_ptr<RendererMainPlatformDelegate> platform_;
   std::unique_ptr<MainFunctionParams> params_;
   std::unique_ptr<base::CommandLine> command_line_;
-  std::unique_ptr<base::FieldTrialList> field_trial_list_;
 
   // For Mojo.
   std::unique_ptr<base::TestIOThread> test_io_thread_;