After https://codereview.chromium.org/2109843003 landed, the Site Isolation Linux FYI bot went red in the following webkit_tests:

https://build.chromium.org/p/chromium.fyi/builders/Site%20Isolation%20Linux/builds/9819
http/tests/multipart/stop-loading.html
http/tests/multipart/stop-loading-after-onload2.html

Looks like it's due to a change in that CL, based on the test output reproduced below.  alexclarke@, can you take a look and either revert or update the test to pass with --site-per-process?  (This bot runs the tests with --site-per-process to turn on out-of-process iframes for cross-site pages.)

14:18:05.404 30708 worker/1 http/tests/multipart/stop-loading.html crashed, (stderr lines):
14:18:05.404 30708   Xlib:  extension "RANDR" missing on display ":9".
14:18:05.404 30708   Xlib:  extension "RANDR" missing on display ":9".
14:18:05.404 30708   [4:4:0706/141805:2331293174:FATAL:web_view_scheduler_impl.cc(135)] Check failed: pending_resource_load_count_ >= 0 (-1 vs. 0)
14:18:05.404 30708   #0 0x0000028c305e base::debug::StackTrace::StackTrace()
14:18:05.404 30708   #1 0x0000028d7f1b logging::LogMessage::~LogMessage()
14:18:05.404 30708   #2 0x000003803a5b scheduler::WebViewSchedulerImpl::decrementPendingResourceLoadCount()
14:18:05.404 30708   #3 0x000001b36050 blink::FrameFetchContext::dispatchDidFail()
14:18:05.404 30708   #4 0x0000019fb19c blink::ResourceFetcher::didFailLoading()
14:18:05.404 30708   #5 0x000001a034bc blink::ResourceLoader::didFail()
14:18:05.404 30708   #6 0x000001a02fbc blink::ResourceLoader::cancel()
14:18:05.404 30708   #7 0x000001a03fc0 blink::ResourceLoaderSet::cancelAll()
14:18:05.404 30708   #8 0x0000019fb5d0 blink::ResourceFetcher::stopFetching()
14:18:05.404 30708   #9 0x000001b3b212 blink::FrameLoader::stopAllLoaders()
14:18:05.404 30708   #10 0x000001355fdd blink::DOMWindowV8Internal::stopMethodCallback()
14:18:05.404 30708   #11 0x00000090977e v8::internal::FunctionCallbackArguments::Call()
14:18:05.404 30708   #12 0x000000997a4b v8::internal::(anonymous namespace)::HandleApiCallHelper<>()
14:18:05.404 30708   #13 0x0000009cd04b v8::internal::Builtin_Impl_HandleApiCall()
14:18:05.404 30708   #14 0x0000009a3fe9 v8::internal::Builtin_HandleApiCall()
14:18:05.404 30708   #15 0x3ebce79063a7 <unknown>
14:18:05.404 30708   
14:18:05.406 30645 [442/3591] http/tests/multipart/stop-loading.html failed unexpectedly (renderer crashed)