[MBI] Add TRACE_EVENTs to show if the ThreadTaskRunnerHandle::Get() returns...

[MBI] Add TRACE_EVENTs to show if the ThreadTaskRunnerHandle::Get() returns default task runner from MTS or AGS.

[related design doc]
https://docs.google.com/document/d/1y-vHkrD1z2RtyWYwT6rJkSLHClYNjDDpUbTtBU7l95A

Bug: 1105403
Change-Id: Iab2c3c9e187f6961f3924222c8ba35c29be82afd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414329
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Tal Pressman <talp@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810512}

base/trace_event/builtin_categories.h

@@ -166,6 +166,7 @@
   X("WebCore")                                                           \
   X("webrtc")                                                            \
   X("xr")                                                                \
+  X(TRACE_DISABLED_BY_DEFAULT("agent_scheduling_group"))                 \
   X(TRACE_DISABLED_BY_DEFAULT("animation-worklet"))                      \
   X(TRACE_DISABLED_BY_DEFAULT("audio"))                                  \
   X(TRACE_DISABLED_BY_DEFAULT("audio-worklet"))                          \

third_party/blink/renderer/platform/scheduler/main_thread/main_thread_scheduler_impl.cc

@@ -2348,7 +2348,21 @@ AgentGroupScheduler* MainThreadSchedulerImpl::GetCurrentAgentGroupScheduler() {
 void MainThreadSchedulerImpl::SetCurrentAgentGroupScheduler(
     AgentGroupSchedulerImpl* agent_group_scheduler_impl) {
   helper_.CheckOnValidThread();
+  if (current_agent_group_scheduler_) {
+    TRACE_EVENT_NESTABLE_ASYNC_END0(
+        TRACE_DISABLED_BY_DEFAULT("agent_scheduling_group"), "ASG_scope", this);
+  } else {
+    TRACE_EVENT_NESTABLE_ASYNC_END0(
+        TRACE_DISABLED_BY_DEFAULT("agent_scheduling_group"), "MTS_scope", this);
+  }
   current_agent_group_scheduler_ = agent_group_scheduler_impl;
+  if (current_agent_group_scheduler_) {
+    TRACE_EVENT_NESTABLE_ASYNC_BEGIN0(
+        TRACE_DISABLED_BY_DEFAULT("agent_scheduling_group"), "ASG_scope", this);
+  } else {
+    TRACE_EVENT_NESTABLE_ASYNC_BEGIN0(
+        TRACE_DISABLED_BY_DEFAULT("agent_scheduling_group"), "MTS_scope", this);
+  }
   if (agent_group_scheduler_impl) {
     sequence_manager_->SetDefaultTaskRunner(
         agent_group_scheduler_impl->DefaultTaskRunner());