Properly reset ProducerClient between TraceEventAgentTest runs

These tests were failing under lsan as the ScopedTaskEnvironment used
in the test environment requires the ProducerClient test runner to be
reset between each test.

R=ssid@chromium.org

Bug: 936907
Change-Id: Iade8c117c43a9071d1e3ea85619930de21f6ea0d
Reviewed-on: https://chromium-review.googlesource.com/c/1495043Reviewed-by: ssid <ssid@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636539}

services/tracing/public/cpp/perfetto/producer_client.cc

@@ -79,6 +79,7 @@ base::SequencedTaskRunner* ProducerClient::GetTaskRunner() {
 
 // static
 void ProducerClient::ResetTaskRunnerForTesting() {
+  DETACH_FROM_SEQUENCE(ProducerClient::Get()->sequence_checker_);
   GetPerfettoTaskRunner()->ResetTaskRunnerForTesting(CreateTaskRunner());
 }
 

services/tracing/public/cpp/trace_event_agent_unittest.cc

@@ -17,6 +17,7 @@
 #include "base/trace_event/trace_event.h"
 #include "base/trace_event/trace_log.h"
 #include "base/values.h"
+#include "services/tracing/public/cpp/perfetto/producer_client.h"
 #include "services/tracing/public/mojom/tracing.mojom.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -82,6 +83,8 @@ class MockRecorder : public mojom::Recorder {
 
 class TraceEventAgentTest : public testing::Test {
  public:
+  void SetUp() override { ProducerClient::ResetTaskRunnerForTesting(); }
+
   void TearDown() override {
     base::trace_event::TraceLog::GetInstance()->SetDisabled();
     recorder_.reset();