Remove one LazyThreadControllerForTest usage of the two

LazyThreadControllerForTest makes it hard to write tests related to task
runners. We aim to remove this class, and this CL is one of the steps.

In media unittests, LazyThreadControllerForTest is not necessary. Use
WebThreadScheduler::CreateMainThreadScheduler instead.

Bug: 870606
Change-Id: I0976c4db02e466f06247bf5a82654734f6c7d7bd
Reviewed-on: https://chromium-review.googlesource.com/1250684Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595725}

media/blink/run_all_unittests.cc

@@ -40,7 +40,8 @@ class BlinkPlatformWithTaskEnvironment : public blink::Platform {
  public:
   BlinkPlatformWithTaskEnvironment()
       : main_thread_scheduler_(
-            blink::scheduler::CreateWebMainThreadSchedulerForTests()) {}
+            blink::scheduler::WebThreadScheduler::CreateMainThreadScheduler()) {
+  }
 
   ~BlinkPlatformWithTaskEnvironment() override {
     main_thread_scheduler_->Shutdown();