[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 32d5915b0c3a008897d0f87e1ca3a3eb0b9f3b74..f9aee4d0cf2ae9cf9b0505e4c78f98b935b4e5e2 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
@@ -227,7 +227,9 @@ 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() {}