Create background tracing agent in all child processes.

The agent was only created in renderer processes earlier.

Change-Id: I7238a73d2c8d9b0ba1c5b52d9acfd4161c5cac91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208081Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771902}

content/browser/browser_child_process_host_impl.cc

@@ -644,6 +644,9 @@ void BrowserChildProcessHostImpl::OnProcessLaunched() {
   tracing_registration_ = TracingServiceController::Get().RegisterClient(
       process.Pid(), base::BindRepeating(&BindTracedProcessFromUIThread,
                                          weak_factory_.GetWeakPtr()));
+  BackgroundTracingManagerImpl::ActivateForProcess(
+      GetData().id,
+      static_cast<ChildProcessHostImpl*>(GetHost())->child_process());
 }
 
 void BrowserChildProcessHostImpl::RegisterCoordinatorClient(