This repros only on an iPad simulator, not on an iPhone simulator (probably just because there's a race condition, not because of the display size). The test crashes with an unhelpful stack:

libc++abi.dylib: terminating with uncaught exception of type std::out_of_range: basic_string
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
  * frame #0: 0x000000010ce16d42 libsystem_kernel.dylib`__pthread_kill + 10
    frame #1: 0x000000010ce4e457 libsystem_pthread.dylib`pthread_kill + 90
    frame #2: 0x000000010cad7107 libsystem_c.dylib`abort + 127
    frame #3: 0x0000000109a4691f libc++abi.dylib`abort_message + 245
    frame #4: 0x0000000109a46aa3 libc++abi.dylib`default_terminate_handler() + 241
    frame #5: 0x0000000109a801c6 libobjc.A.dylib`_objc_terminate() + 105
    frame #6: 0x0000000109a62159 libc++abi.dylib`std::__terminate(void (*)()) + 8
    frame #7: 0x0000000109a61e0a libc++abi.dylib`__cxa_rethrow + 99
    frame #8: 0x0000000109a800dc libobjc.A.dylib`objc_exception_rethrow + 40
    frame #9: 0x0000000106c3cb09 CoreFoundation`CFRunLoopRunSpecific + 537
    frame #10: 0x000000010b0c89c6 GraphicsServices`GSEventRunModal + 62
    frame #11: 0x0000000104a7ad30 UIKit`UIApplicationMain + 159
    frame #12: 0x00000001007e7d22 ios_web_inttests`base::RunTestsFromIOSApp() at test_support_ios.mm:213
    frame #13: 0x00000001007e36ef ios_web_inttests`base::TestSuite::Run(this=0x00007fff5f9c3388) at test_suite.cc:251

Stepping through the test, this seems to happen during or just after the ASSERT_TRUE call.

However, if I set a breakpoint in TestUIHandler::HandleJsMessage (on line 73 of web_ui_mojo_inttest.mm), the test passes.

So there's some race that needs further investigation.

The test loads a WebUI page and expects to receive a Mojo message from the page.