Add experiment to turn on responsiveness calculator.

The logic is implemented for Windows, macOS and Linux. It may add non-neglible
overhead, since it adds calls to base::TimeTicks::Now() to every posted task. It
will be turned on behind an experiment so that we can measure the impact.

Bug: 859155
Change-Id: I47c2a4629470d892dcb416a3b2121ef37b87ac8c
Reviewed-on: https://chromium-review.googlesource.com/1172913Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582697}

content/browser/browser_main_loop.cc

@@ -83,6 +83,7 @@
 #include "content/browser/net/browser_online_state_observer.h"
 #include "content/browser/renderer_host/media/media_stream_manager.h"
 #include "content/browser/renderer_host/render_process_host_impl.h"
+#include "content/browser/scheduler/responsiveness/watcher.h"
 #include "content/browser/service_manager/service_manager_context.h"
 #include "content/browser/speech/speech_recognition_manager_impl.h"
 #include "content/browser/startup_data_impl.h"
@@ -437,6 +438,9 @@ void SetFileUrlPathAliasForIpcFuzzer() {
 }
 #endif
 
+const base::Feature kBrowserResponsivenessCalculator{
+    "BrowserResponsivenessCalculator", base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace
 
 #if defined(USE_X11)
@@ -1472,6 +1476,11 @@ int BrowserMainLoop::BrowserThreadsStarted() {
   SystemHotkeyHelperMac::GetInstance()->DeferredLoadSystemHotkeys();
 #endif  // defined(OS_MACOSX)
 
+  if (base::FeatureList::IsEnabled(kBrowserResponsivenessCalculator)) {
+    responsiveness_watcher_ = new responsiveness::Watcher;
+    responsiveness_watcher_->SetUp();
+  }
+
 #if defined(OS_ANDROID)
   media::SetMediaDrmBridgeClient(GetContentClient()->GetMediaDrmBridgeClient());
 #endif

content/browser/browser_main_loop.h

@@ -102,6 +102,10 @@ class SwapMetricsDriver;
 class TracingControllerImpl;
 struct MainFunctionParams;
 
+namespace responsiveness {
+class Watcher;
+}  // namespace responsiveness
+
 #if defined(OS_ANDROID)
 class ScreenOrientationDelegate;
 #endif
@@ -370,6 +374,7 @@ class CONTENT_EXPORT BrowserMainLoop {
       discardable_shared_memory_manager_;
   scoped_refptr<SaveFileManager> save_file_manager_;
   std::unique_ptr<content::TracingControllerImpl> tracing_controller_;
+  scoped_refptr<responsiveness::Watcher> responsiveness_watcher_;
 #if !defined(OS_ANDROID)
   // A SharedBitmapManager used to sharing and mapping IDs to shared memory
   // between processes for software compositing. When the display compositor is