VCD: Start tracing in CameraHalDispatcherImpl::Start()

There is a race condition in how we added tracing in
https://crrev.com/c/1158106
TraceLog is added in CameraHalDispatcherImpl constructor. Once TraceLog
is enabled, it calls CameraHalDispatcherImpl::OnTraceLogEnabled.
CameraHalDispatcherImpl then processes the call on its proxy thread.
The issue is the proxy thread may not be started yet at this point, so
CameraHalDispatcherImpl will crash when it tries to post a task onto the
not-yet-started proxy thread.

Bug: None
Test: Build and deploy simplechrome on soraka

Change-Id: If06c18da2aac25aae8ff79d061fd59b3344e9369
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2581125
Commit-Queue: Jasmine Chen <lnishan@google.com>
Commit-Queue: Shik Chen <shik@chromium.org>
Auto-Submit: Jasmine Chen <lnishan@google.com>
Reviewed-by: Shik Chen <shik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835124}

media/capture/video/chromeos/camera_hal_dispatcher_impl.cc

@@ -144,6 +144,10 @@ bool CameraHalDispatcherImpl::Start(
   if (!StartThreads()) {
     return false;
   }
+  // This event is for adding camera category to categories list.
+  TRACE_EVENT0("camera", "CameraHalDispatcherImpl");
+  base::trace_event::TraceLog::GetInstance()->AddEnabledStateObserver(this);
+
   jda_factory_ = std::move(jda_factory);
   jea_factory_ = std::move(jea_factory);
   base::WaitableEvent started(base::WaitableEvent::ResetPolicy::MANUAL,
@@ -218,11 +222,7 @@ CameraHalDispatcherImpl::CameraHalDispatcherImpl()
       blocking_io_thread_("CameraBlockingIOThread"),
       camera_hal_server_callbacks_(this),
       active_client_observers_(
-          new base::ObserverListThreadSafe<CameraActiveClientObserver>()) {
-  // This event is for adding camera category to categories list.
-  TRACE_EVENT0("camera", "CameraHalDispatcherImpl");
-  base::trace_event::TraceLog::GetInstance()->AddEnabledStateObserver(this);
-}
+          new base::ObserverListThreadSafe<CameraActiveClientObserver>()) {}
 
 CameraHalDispatcherImpl::~CameraHalDispatcherImpl() {
   VLOG(1) << "Stopping CameraHalDispatcherImpl...";
@@ -233,7 +233,6 @@ CameraHalDispatcherImpl::~CameraHalDispatcherImpl() {
     proxy_thread_.Stop();
   }
   blocking_io_thread_.Stop();
-  base::trace_event::TraceLog::GetInstance()->RemoveEnabledStateObserver(this);
   VLOG(1) << "CameraHalDispatcherImpl stopped";
 }
 
@@ -595,6 +594,7 @@ void CameraHalDispatcherImpl::OnCameraHalClientConnectionError(
 
 void CameraHalDispatcherImpl::StopOnProxyThread() {
   DCHECK(proxy_task_runner_->BelongsToCurrentThread());
+  base::trace_event::TraceLog::GetInstance()->RemoveEnabledStateObserver(this);
 
   // TODO(crbug.com/1053569): Remove these lines once the issue is solved.
   base::File::Info info;