Use ScopedTaskEnvironment instead of MessageLoop in /ui/ozone/platform/drm

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=dcastagna@chromium.org

Change-Id: I92a1b8c3530059938c9300774d0a9f3e007b49c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649324
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667161}

ui/ozone/platform/drm/gpu/proxy_helpers_unittest.cc

@@ -8,8 +8,8 @@
 #include <utility>
 
 #include "base/bind.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_checker_impl.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -32,7 +32,7 @@ class ProxyHelpersTest : public testing::Test {
   void QuitFunction(int a) {
     EXPECT_TRUE(drm_checker_.CalledOnValidThread());
 
-    message_loop_.task_runner()->PostTask(
+    scoped_task_environment_.GetMainThreadTaskRunner()->PostTask(
         FROM_HERE, base::BindOnce(&ProxyHelpersTest::QuitFunctionCallback,
                                   base::Unretained(this), 8));
   }
@@ -42,7 +42,8 @@ class ProxyHelpersTest : public testing::Test {
     EXPECT_TRUE(main_checker_.CalledOnValidThread());
 
     auto quitter = run_loop_.QuitWhenIdleClosure();
-    message_loop_.task_runner()->PostTask(FROM_HERE, quitter);
+    scoped_task_environment_.GetMainThreadTaskRunner()->PostTask(FROM_HERE,
+                                                                 quitter);
   }
 
   void SetDrmChecker() { drm_checker_.DetachFromThread(); }
@@ -90,7 +91,7 @@ class ProxyHelpersTest : public testing::Test {
 
  protected:
   // Main thread message loop.
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   base::RunLoop run_loop_;
 
   // Thread to simulate the drm thread in ozone viz process.