[Tracing] Start Java tracing only after native tracing has started

Enabling Java TraceEvents when the tracing session has been only set up
(but not started) leads to weird data races in tests (see the bug).

This CL turns on Java events after the tracing has started and
re-enables the flaky test.

Bug: 1472381
Change-Id: Ic40e6fb647b45822d3d0290a223b2dfe0564fad3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4793489
Commit-Queue: Mikhail Khokhlov <khokhlov@google.com>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1185150}

base/android/trace_event_binding.cc

@@ -43,7 +43,9 @@ class TraceEnabledObserver : public perfetto::TrackEventSessionObserver {
         args.config->chrome_config().trace_config());
     event_name_filtering_per_session_[args.internal_instance_index] =
         trace_config.IsEventPackageNameFilterEnabled();
+  }
 
+  void OnStart(const perfetto::DataSourceBase::StartArgs&) override {
     JNIEnv* env = base::android::AttachCurrentThread();
     base::android::Java_TraceEvent_setEnabled(env, true);
     base::android::Java_TraceEvent_setEventNameFilteringEnabled(

content/browser/tracing/background_tracing_manager_browsertest.cc

@@ -1737,13 +1737,7 @@ IN_PROC_BROWSER_TEST_F(ProtoBackgroundTracingTest,
   background_tracing_helper.WaitForScenarioIdle();
 }
 
-// TODO(https://crbug.com/1472381): Flaky on Android
-#if BUILDFLAG(IS_ANDROID)
-#define MAYBE_ProtoTraceReceived DISABLED_ProtoTraceReceived
-#else
-#define MAYBE_ProtoTraceReceived ProtoTraceReceived
-#endif
-IN_PROC_BROWSER_TEST_F(ProtoBackgroundTracingTest, MAYBE_ProtoTraceReceived) {
+IN_PROC_BROWSER_TEST_F(ProtoBackgroundTracingTest, ProtoTraceReceived) {
   TestBackgroundTracingHelper background_tracing_helper;
 
   std::unique_ptr<BackgroundTracingConfig> config = CreatePreemptiveConfig();