This test appears to always require a retry, whereupon it always succeeds, suggesting pollution of global state by some earlier test in the same process. Crash stack looks like:

[15039:15039:1204/140831.036724:1741590998125:FATAL:FrameTestHelpers.cpp(334)] Check failed: !TestWebFrameClient::IsLoading(). 
#0 0x7fe672a95b9d base::debug::StackTrace::StackTrace()
#1 0x7fe672a93fcc base::debug::StackTrace::StackTrace()
#2 0x7fe672b1ba6a logging::LogMessage::~LogMessage()
#3 0x000001b836f8 blink::FrameTestHelpers::WebViewHelper::Reset()
#4 0x000001b83569 blink::FrameTestHelpers::WebViewHelper::~WebViewHelper()
#5 0x000002443078 blink::SimTest::~SimTest()
#6 0x000001f0e0af blink::(anonymous namespace)::ScrollbarAppearanceTest::~ScrollbarAppearanceTest()
#7 0x000001f0d755 blink::(anonymous namespace)::ScrollbarAppearanceTest_ThemeEngineDefinesMinimumThumbLength_Test::~ScrollbarAppearanceTest_ThemeEngineDefinesMinimumThumbLength_Test()
#8 0x000001f0d779 blink::(anonymous namespace)::ScrollbarAppearanceTest_ThemeEngineDefinesMinimumThumbLength_Test::~ScrollbarAppearanceTest_ThemeEngineDefinesMinimumThumbLength_Test()
#9 0x000000a5986b WTF::RefCounted<>::DeleteInternal<>()
#10 0x000000be978e testing::internal::HandleSehExceptionsInMethodIfSupported<>()
#11 0x000000bd9d72 testing::internal::HandleExceptionsInMethodIfSupported<>()
#12 0x000000bc4e75 testing::TestInfo::Run()
#13 0x000000bc588c testing::TestCase::Run()
#14 0x000000bd15cc testing::internal::UnitTestImpl::RunAllTests()
#15 0x000000be989e testing::internal::HandleSehExceptionsInMethodIfSupported<>()
#16 0x000000bdb412 testing::internal::HandleExceptionsInMethodIfSupported<>()
#17 0x000000bd120e testing::UnitTest::Run()
#18 0x000002a9ab71 RUN_ALL_TESTS()
#19 0x000002a97b92 base::TestSuite::Run()
#20 0x0000009f22b2 (anonymous namespace)::runHelper()
#21 0x0000009f3785 _ZN4base8internal13FunctorTraitsIPFiPNS_9TestSuiteEEvE6InvokeIJS3_EEEiS5_DpOT_
#22 0x0000009f3720 _ZN4base8internal12InvokeHelperILb0EiE8MakeItSoIRKPFiPNS_9TestSuiteEEJS5_EEEiOT_DpOT0_
#23 0x0000009f36e5 _ZN4base8internal7InvokerINS0_9BindStateIPFiPNS_9TestSuiteEEJNS0_17UnretainedWrapperIS3_EEEEEFivEE7RunImplIRKS6_RKNSt3__15tupleIJS8_EEEJLm0EEEEiOT_OT0_NSF_16integer_sequenceImJXspT1_EEEE
#24 0x0000009f362c _ZN4base8internal7InvokerINS0_9BindStateIPFiPNS_9TestSuiteEEJNS0_17UnretainedWrapperIS3_EEEEEFivEE3RunEPNS0_13BindStateBaseE
#25 0x000000c0575d _ZNKR4base17RepeatingCallbackIFvvEE3RunEv
#26 0x000002aa71c7 base::(anonymous namespace)::LaunchUnitTestsInternal()
#27 0x000002aa7035 base::LaunchUnitTests()
#28 0x0000009f2242 main
#29 0x7fe65fd882b1 __libc_start_main
#30 0x0000009f102a _start