[tracing] Implement Add/RemoveTraceStateObserver for default platform.

src/libplatform/tracing/tracing-controller.cc

Index: src/libplatform/tracing/tracing-controller.cc
diff --git a/src/libplatform/tracing/tracing-controller.cc b/src/libplatform/tracing/tracing-controller.cc
index e9a21725e289fd565391084a0b933a00c82df407..4d9321436508629ff02392404d7ff6aea80c94d0 100644
--- a/src/libplatform/tracing/tracing-controller.cc
+++ b/src/libplatform/tracing/tracing-controller.cc
@@ -38,8 +38,13 @@ const int g_num_builtin_categories = 4;
 // Skip default categories.
 v8::base::AtomicWord g_category_index = g_num_builtin_categories;
 
+TracingController::TracingController() {}
+
+TracingController::~TracingController() {}
+
 void TracingController::Initialize(TraceBuffer* trace_buffer) {
   trace_buffer_.reset(trace_buffer);
+  mutex_.reset(new base::Mutex());
 }
 
 uint64_t TracingController::AddTraceEvent(
@@ -91,13 +96,29 @@ const char* TracingController::GetCategoryGroupName(
 
 void TracingController::StartTracing(TraceConfig* trace_config) {
   trace_config_.reset(trace_config);
-  mode_ = RECORDING_MODE;
-  UpdateCategoryGroupEnabledFlags();
+  std::unordered_set<Platform::TraceStateObserver*> observers_copy;
+  {
+    base::LockGuard<base::Mutex> lock(mutex_.get());
+    mode_ = RECORDING_MODE;
+    UpdateCategoryGroupEnabledFlags();
+    observers_copy = observers_;
+  }
+  for (auto o : observers_copy) {
+    o->OnTraceEnabled();
+  }
 }
 
 void TracingController::StopTracing() {
   mode_ = DISABLED;
   UpdateCategoryGroupEnabledFlags();
+  std::unordered_set<Platform::TraceStateObserver*> observers_copy;
+  {
+    base::LockGuard<base::Mutex> lock(mutex_.get());
+    observers_copy = observers_;
+  }
+  for (auto o : observers_copy) {
+    o->OnTraceDisabled();
+  }
   trace_buffer_->Flush();
 }
 
@@ -172,6 +193,24 @@ const uint8_t* TracingController::GetCategoryGroupEnabledInternal(
   return category_group_enabled;
 }
 
+void TracingController::AddTraceStateObserver(
+    Platform::TraceStateObserver* observer) {
+  {
+    base::LockGuard<base::Mutex> lock(mutex_.get());
+    observers_.insert(observer);
+    if (mode_ != RECORDING_MODE) return;
+  }
+  // Fire the observer if recording is already in progress.
+  observer->OnTraceEnabled();
+}
+
+void TracingController::RemoveTraceStateObserver(
+    Platform::TraceStateObserver* observer) {
+  base::LockGuard<base::Mutex> lock(mutex_.get());
+  DCHECK(observers_.find(observer) != observers_.end());
+  observers_.erase(observer);
+}
+
 }  // namespace tracing
 }  // namespace platform
 }  // namespace v8