[scheduler] Disable background thread load tracker by default.

third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc

Index: third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc
diff --git a/third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc b/third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc
index 1e7b4e5cd8fa786eacdb6d94ff2d9b5d25a534f4..7de5986af7408997f8fd95f72d98bf9657d5989e 100644
--- a/third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc
+++ b/third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc
@@ -223,7 +223,9 @@ RendererSchedulerImpl::MainThreadOnly::MainThreadOnly(
       in_idle_period_for_testing(false),
       use_virtual_time(false),
       is_audio_playing(false),
-      rail_mode_observer(nullptr) {}
+      rail_mode_observer(nullptr) {
+  foreground_main_thread_load_tracker.Resume(now);
+}
 
 RendererSchedulerImpl::MainThreadOnly::~MainThreadOnly() {}