Use ScopedTaskEnvironment instead of MessageLoop in /chromecast

MessageLoop will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you belive your test needs to make sure
that no ThreadPool runs let me know and I will update the patch.

BUG=891670
This CL was uploaded by git cl split.

R=yucliu@chromium.org

Change-Id: Idbad32ad54c52846e28dbd56841e4c97b8c44fc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649170
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667194}

chromecast/base/bind_to_task_runner_unittest.cc

@@ -10,8 +10,8 @@
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -50,7 +50,7 @@ class BindToTaskRunnerTest : public ::testing::Test {
  public:
   ~BindToTaskRunnerTest() override { base::RunLoop().RunUntilIdle(); }
 
-  base::MessageLoop loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   MockCallbacks callbacks_;
 };