Fix ios_chrome_unittests to crash on DCHECK failure.

ios/chrome/browser/metrics/first_user_action_recorder.cc

Index: ios/chrome/browser/metrics/first_user_action_recorder.cc
diff --git a/ios/chrome/browser/metrics/first_user_action_recorder.cc b/ios/chrome/browser/metrics/first_user_action_recorder.cc
index d93836cd57efc69cc5476c97874c02f8440c2e1e..aac5e0cf64dc86232b9af3a6e777c54b8dac68c6 100644
--- a/ios/chrome/browser/metrics/first_user_action_recorder.cc
+++ b/ios/chrome/browser/metrics/first_user_action_recorder.cc
@@ -91,8 +91,6 @@ FirstUserActionRecorder::FirstUserActionRecorder(
       background_duration_(background_duration),
       action_callback_(base::Bind(&FirstUserActionRecorder::OnUserAction,
                                   base::Unretained(this))) {
-  base::SetRecordActionTaskRunner(
-      web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));
   base::AddActionCallback(action_callback_);
 }