Use ScopedTaskEnvironment instead of MessageLoopForUI in remoting tests.

ScopedTaskEnvironment allows usage of ThreadTaskRunnerHandle and
base/task_scheduler/post_task.h within its scope. It should be
instantiated in everytest that uses either of these APIs
(i.e. no test should instantiate a MessageLoop directly).

Motivation for ScopedTaskEnvironment can be found in:
https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit

BUG=708584
R=gab@chromium.org
TBR=garykac@chromium.org

Review-Url: https://codereview.chromium.org/2846943002
Cr-Commit-Position: refs/heads/master@{#468018}

remoting/test/BUILD.gn

@@ -173,6 +173,7 @@ if (enable_remoting_host && !is_android && !is_chromeos) {
 
     deps = [
       ":fake_connection_event_logger",
+      "//base/test:test_support",
       "//third_party/webrtc/modules/desktop_capture",
     ]
 

remoting/test/it2me_standalone_host.cc

@@ -12,6 +12,7 @@
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #include "remoting/base/auto_thread_task_runner.h"
 #include "remoting/host/chromoting_host_context.h"
@@ -36,9 +37,11 @@ constexpr char kSessionJid[] = "user@domain/rest-of-jid";
 using ::remoting::protocol::MockSession;
 
 It2MeStandaloneHost::It2MeStandaloneHost()
-  : context_(ChromotingHostContext::Create(
-        new AutoThreadTaskRunner(
-        message_loop_.task_runner(), run_loop_.QuitClosure()))),
+    : scoped_task_environment_(
+          base::test::ScopedTaskEnvironment::MainThreadType::UI),
+      context_(ChromotingHostContext::Create(
+          new AutoThreadTaskRunner(base::ThreadTaskRunnerHandle::Get(),
+                                   run_loop_.QuitClosure()))),
       main_task_runner_(context_->file_task_runner()),
       factory_(main_task_runner_,
                context_->video_capture_task_runner(),

remoting/test/it2me_standalone_host.h

@@ -9,8 +9,8 @@
 #include <string>
 
 #include "base/memory/ref_counted.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/timer/timer.h"
 #include "remoting/host/host_mock_objects.h"
 #include "remoting/host/it2me_desktop_environment.h"
@@ -43,7 +43,7 @@ class It2MeStandaloneHost {
  private:
   void Connect();
 
-  base::MessageLoopForUI message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   base::RunLoop run_loop_;
   std::unique_ptr<ChromotingHostContext> context_;
   scoped_refptr<AutoThreadTaskRunner> main_task_runner_;