I could reproduce locally by invoking the test in this fashion:
autoninja -C out/fuchsia cronet_tests && out/fuchsia/bin/run_cronet_tests --gtest_filter=UrlRequestTest.GetStatus/1 --gtest_repeat=1000 --gtest_break_on_failure --test-launcher-jobs=32

Sample stack trace:
[ RUN      ] UrlRequestTest.GetStatus/1
[174885:665684971:0114/200531.377575:53777477:FATAL:engine.cc(263)] Check failed: Cronet_RESULT_SUCCESS == result (0 vs. -209)
#00: base::debug::StackTrace::StackTrace(unsigned long) at ../../base/debug/stack_trace_fuchsia.cc:175
#01: endl<char, std::__1::char_traits<char> > at ../../buildtools/third_party/libc++/trunk/include/ostream:1002
      (inlined by) operator<< at ../../buildtools/third_party/libc++/trunk/include/ostream:196
      (inlined by) ~LogMessage at ../../base/logging.cc:592
#02: cronet::Cronet_EngineImpl::CheckResult(Cronet_RESULT) at ../../components/cronet/native/engine.cc:264
#03: cronet::Cronet_UrlRequestImpl::Read(Cronet_Buffer*) at url_request.cc:?
#04: Cronet_UrlRequest_Read at ../../components/cronet/native/generated/cronet.idl_impl_interface.cc:787
#05: (anonymous namespace)::UrlRequestTest_GetStatus_Test::TestBody() at ../../components/cronet/native/test/url_request_test.cc:1045
#06: testing::Test::Run() at ../../third_party/googletest/src/googletest/src/gtest.cc:2529
#07: testing::TestInfo::Run() at ../../third_party/googletest/src/googletest/src/gtest.cc:2707
#08: testing::TestCase::Run() at ../../third_party/googletest/src/googletest/src/gtest.cc:2824
#09: testing::internal::UnitTestImpl::RunAllTests() at ../../third_party/googletest/src/googletest/src/gtest.cc:5226
#10: testing::UnitTest::Run() at ../../third_party/googletest/src/googletest/src/gtest.cc:4835
#11: base::TestSuite::Run() at ../../base/test/test_suite.cc:294
#12: int base::internal::Invoker<base::internal::BindState<int (base::TestSuite::*)(), base::internal::UnretainedWrapper<base::TestSuite> >, int ()>::RunImpl<int (base::TestSuite::*)(), std::__1::tuple<base::internal::UnretainedWrapper<base::TestSuite> >, 0ul>(int (base::TestSuite::*&&)(), std::__1::tuple<base::internal::UnretainedWrapper<base::TestSuite> >&&, std::__1::integer_sequence<unsigned long, 0ul>) at ../../base/bind_internal.h:689
#13: base::OnceCallback<bool ()>::Run() && at ../../base/callback.h:99
#14: base::(anonymous namespace)::LaunchUnitTestsInternal(base::OnceCallback<int ()>, unsigned long, int, bool, base::OnceCallback<void ()>) at ../../base/test/launcher/unit_test_launcher.cc:225
#15: base::LaunchUnitTests(int, char**, base::OnceCallback<int ()>) at ../../base/test/launcher/unit_test_launcher.cc:575
#16: main at ../../components/cronet/run_all_unittests.cc:17
#17: pc 0x4a1c20a3bf68 (libc.so,0x19f68)

This sometimes triggers this check in Cronet_EngineImpl::CheckResult in engine.cc: CHECK_EQ(Cronet_RESULT_SUCCESS, result), indicating the Read request was unsuccessful. I am not sure what "-209" refers to there, though.