[scheduler] Making MessageLoop backend variant dependent on Finch config.

We want to A/B test two different backends for MessageLoop: (old) MessageLoopImpl and
(new) SequenceManager, as part of an ongoing effort to get rid of MessageLoop
completely. In order to use Finch, we move its initialisation before that of
MessageLoop in all builds and tests.

Bug: 891670
Change-Id: I808f72b6cf36238f7a2380568a0898ed6c0f5557

Forcing due to fuschia_x64 tests failing because of infrastructure flakiness.
NOTRY=true

Change-Id: I808f72b6cf36238f7a2380568a0898ed6c0f5557
Reviewed-on: https://chromium-review.googlesource.com/c/1333770
Commit-Queue: Karolina Soltys <ksolt@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610876}

base/message_loop/message_loop.cc

@@ -12,6 +12,7 @@
 #include "base/callback_helpers.h"
 #include "base/compiler_specific.h"
 #include "base/debug/task_annotator.h"
+#include "base/feature_list.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop_impl.h"
@@ -183,10 +184,15 @@ std::unique_ptr<MessageLoop> MessageLoop::CreateUnbound(
   return WrapUnique(new MessageLoop(type, std::move(pump_factory)));
 }
 
+const Feature kMessageLoopUsesSequenceManager{"MessageLoopUsesSequenceManager",
+                                              FEATURE_DISABLED_BY_DEFAULT};
+
 MessageLoop::MessageLoop(Type type, MessagePumpFactoryCallback pump_factory)
     : MessageLoop(type,
                   std::move(pump_factory),
-                  BackendType::MESSAGE_LOOP_IMPL) {}
+                  FeatureList::IsEnabled(kMessageLoopUsesSequenceManager)
+                      ? BackendType::SEQUENCE_MANAGER
+                      : BackendType::MESSAGE_LOOP_IMPL) {}
 
 MessageLoop::MessageLoop(Type type,
                          MessagePumpFactoryCallback pump_factory,

content/app/content_main_runner_impl.cc

@@ -858,6 +858,12 @@ int ContentMainRunnerImpl::Run(bool start_service_manager_only) {
   // BrowserMain() which may elect to promote it (e.g. to BrowserThread::IO).
   if (process_type.empty()) {
     startup_data_ = std::make_unique<StartupDataImpl>();
+
+    if (delegate_->ShouldCreateFeatureList()) {
+      DCHECK(!field_trial_list_);
+      field_trial_list_ = SetUpFieldTrialsAndFeatureList();
+    }
+
     startup_data_->thread = BrowserProcessSubThread::CreateIOThread();
     main_params.startup_data = startup_data_.get();
 
@@ -883,11 +889,6 @@ int ContentMainRunnerImpl::Run(bool start_service_manager_only) {
     if (!base::MessageLoopCurrentForUI::IsSet())
       main_message_loop_ = std::make_unique<base::MessageLoopForUI>();
 
-    if (delegate_->ShouldCreateFeatureList()) {
-      DCHECK(!field_trial_list_);
-      field_trial_list_ = SetUpFieldTrialsAndFeatureList();
-    }
-
     delegate_->PostEarlyInitialization(main_params.ui_task != nullptr);
 
     if (GetContentClient()->browser()->ShouldCreateTaskScheduler()) {

gpu/command_buffer/tests/gl_tests_main.cc

@@ -21,12 +21,12 @@
 namespace {
 
 int RunHelper(base::TestSuite* testSuite) {
+  base::FeatureList::InitializeInstance(std::string(), std::string());
 #if defined(USE_OZONE)
   base::MessageLoopForUI main_loop;
 #else
   base::MessageLoopForIO message_loop;
 #endif
-  base::FeatureList::InitializeInstance(std::string(), std::string());
   gpu::GLTestHelper::InitializeGLDefault();
 
   ::gles2::Initialize();

gpu/perftests/run_all_tests.cc

@@ -12,13 +12,12 @@
 #include "ui/gl/init/gl_factory.h"
 
 static int RunHelper(base::TestSuite* test_suite) {
+  base::FeatureList::InitializeInstance(std::string(), std::string());
 #if defined(USE_OZONE)
   base::MessageLoopForUI main_loop;
 #else
   base::MessageLoopForIO message_loop;
 #endif
-  base::FeatureList::InitializeInstance(std::string(), std::string());
-
   CHECK(gl::init::InitializeGLOneOff());
   return test_suite->Run();
 }

ppapi/nacl_irt/ppapi_dispatcher.cc

@@ -182,6 +182,7 @@ void PpapiDispatcher::OnMsgInitializeNaClDispatcher(
   settings.logging_dest = logging::LOG_TO_SYSTEM_DEBUG_LOG;
   logging::InitLogging(settings);
 
+  base::FeatureList::ClearInstanceForTesting();
   base::FeatureList::InitializeInstance(
       base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
           switches::kEnableFeatures),

remoting/test/chromoting_test_driver.cc

@@ -150,8 +150,8 @@ void PrintJsonFileInfo() {
 
 int main(int argc, char* argv[]) {
   base::TestSuite test_suite(argc, argv);
-  base::MessageLoopForIO message_loop;
   base::FeatureList::InitializeInstance(std::string(), std::string());
+  base::MessageLoopForIO message_loop;
 
   if (!base::CommandLine::InitializedForCurrentProcess()) {
     if (!base::CommandLine::Init(argc, argv)) {