message: Switch from GTEST_FLAG to GTEST_FLAG_SET.
  testing::GTEST_FLAG is deprecated and it will be removed in future
  versions of gtest.
  Cr-Commit-Position: refs/heads/master@{#34600}
diff: @@ -446,7 +446,7 @@ TEST(ThreadTest, Invoke) {
  // not deadlock but crash.
  #if RTC_DCHECK_IS_ON && GTEST_HAS_DEATH_TEST && !defined(WEBRTC_ANDROID)
  TEST(ThreadTest, TwoThreadsInvokeDeathTest) {
  - ::testing::GTEST_FLAG(death_test_style) = "threadsafe";
  + GTEST_FLAG_SET(death_test_style, "threadsafe");
  AutoThread thread;
  Thread* main_thread = Thread::Current();
  auto other_thread = Thread::CreateWithSocketServer();
  @@ -457,7 +457,7 @@ TEST(ThreadTest, TwoThreadsInvokeDeathTest) {
  }
  TEST(ThreadTest, ThreeThreadsInvokeDeathTest) {
  - ::testing::GTEST_FLAG(death_test_style) = "threadsafe";
  + GTEST_FLAG_SET(death_test_style, "threadsafe");
  AutoThread thread;
  Thread* first = Thread::Current();
  