Findit identified the culprit r574398 with confidence 70.0% in the config "chromium.linux / Linux Tests"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVywgELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKLAWNocm9taXVtLmxpbnV4L0xpbnV4IFRlc3RzLzcwOTg4L3Zpel9icm93c2VyX3Rlc3RzL1UzUmhjblIxY0VKeWIzZHpaWEpEY21WaGRHOXlWMlZzWTI5dFpVSmhZMnRVWlhOMExsZGxiR052YldWQ1lXTnJVM1JoYm1SaGNtUk9iMUJ2YkdsamVRPT0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

If the culprit above is wrong, please file a bug using this link and hit submit:
https://bugs.chromium.org/p/chromium/issues/entry?status=Unconfirmed&labels=Pri-1,Test-Findit-Wrong&components=Tools%3ETest%3EFindit%3EFlakiness&summary=%5BFindit%5D%20Flake%20Analyzer%20-%20Wrong%20result%20for%20StartupBrowserCreatorWelcomeBackTest.WelcomeBackStandardNoPolicy&comment=Link%20to%20Analysis%3A%20https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVywgELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKLAWNocm9taXVtLmxpbnV4L0xpbnV4IFRlc3RzLzcwOTg4L3Zpel9icm93c2VyX3Rlc3RzL1UzUmhjblIxY0VKeWIzZHpaWEpEY21WaGRHOXlWMlZzWTI5dFpVSmhZMnRVWlhOMExsZGxiR052YldWQ1lXTnJVM1JoYm1SaGNtUk9iMUJ2YkdsamVRPT0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).

BrowserTestBase received signal: Segmentation fault. Backtrace:
#0 0x000004f3d20c base::debug::StackTrace::StackTrace()
#1 0x0000053a6965 content::(anonymous namespace)::DumpStackTraceSignalHandler()
#2 0x7f3e8caf8cb0 <unknown>
#3 0x000001f68a00 std::__1::__tree<>::find<>()
#4 0x000005b7cde5 ui::ReplaceTemplateExpressions()
#5 0x000005b7d827 ui::I18nSourceStream::FilterData()
#6 0x0000054c508e net::FilterSourceStream::DoFilterData()
#7 0x0000054c4d18 net::FilterSourceStream::DoLoop()
#8 0x0000054c5133 net::FilterSourceStream::OnIOComplete()
#9 0x00000561955b net::URLRequestJob::ReadRawDataComplete()
#10 0x000003d8d3de _ZN4base8internal7InvokerINS0_9BindStateIZN7content8protocol12_GLOBAL__N_114SetCookiesOnIOEPN3net23URLRequestContextGetterENSt3__16vectorINS9_10unique_ptrINS6_15CanonicalCookieENS9_14default_deleteISC_EEEENS9_9allocatorISF_EEEENS_12OnceCallbackIFvvEEEE3$_4JNS_17RepeatingCallbackISK_EEEEEFvbEE7RunOnceEPNS0_13BindStateBaseEb
#11 0x000004f1407a _ZN4base8internal7InvokerINS0_9BindStateIPFvNS_12_GLOBAL__N_121PostTaskAndReplyRelayEEJS4_EEEFvvEE7RunOnceEPNS0_13BindStateBaseE
#12 0x000004ec1459 base::debug::TaskAnnotator::RunTask()
#13 0x000004ec0452 base::MessageLoop::RunTask()
#14 0x000004ec0987 base::MessageLoop::DoWork()
#15 0x000004f4e659 base::MessagePumpLibevent::Run()
#16 0x000004ee0395 base::RunLoop::Run()
#17 0x000003d13944 content::BrowserProcessSubThread::IOThreadRun()
#18 0x000004f15c9f base::Thread::ThreadMain()
#19 0x000004f4a0ad base::(anonymous namespace)::ThreadFunc()
#20 0x7f3e92766184 start_thread
#21 0x7f3e8cbc003d clone
[1/1] StartupBrowserCreatorWelcomeBackTest.WelcomeBackStandardNoPolicy (CRASHED)

Assign to Chris to take a look first based on Findit.

Going to disable the test:
https://chromium-review.googlesource.com/c/chromium/src/+/1138406

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ca5fb9e3c57825716e620ac67e73a637a7fb03e1

commit ca5fb9e3c57825716e620ac67e73a637a7fb03e1
Author: Owen Min <zmin@chromium.org>
Date: Mon Jul 16 16:16:36 2018

Disable StartupBrowserCreatorWelcomeBackTest.WelcomeBackStandardNoPolicy on Linux

Tbr: tmartino
Bug:  863743 
Change-Id: I732c8f932cdc9be220b35c8d78f19bbc54e44dcb
Reviewed-on: https://chromium-review.googlesource.com/1138406
Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575293}
[modify] https://crrev.com/ca5fb9e3c57825716e620ac67e73a637a7fb03e1/chrome/browser/ui/startup/startup_browser_creator_browsertest.cc

This is happening across all desktop platforms. Please take a look.

Here's a Win crash:

https://chromium-swarm.appspot.com/task?id=3ede697f36cce710&refresh=10&show_raw=1

[ RUN      ] StartupBrowserCreatorWelcomeBackTest.WelcomeBackStandardNoPolicy
[8432:6432:0722/222713.131:WARNING:discovery_network_list_win.cc(195)] Failed to open Wlan client handle: 1062
[8432:6880:0722/222713.147:WARNING:chrome_browser_main_win.cc(653)] Command line too long for RegisterApplicationRestart:  --brave-new-test-launcher --cfi-diag=0 --disable-gpu-process-for-dx12-vulkan-info-collection --gtest_also_run_disabled_tests --gtest_filter=StartupBrowserCreatorWelcomeBackTest.WelcomeBackStandardNoPolicy --single_process --test-launcher-bot-mode --test-launcher-output="C:\Users\CHROME~2\AppData\Local\Temp\scoped_dir6552_8905\results6552_17789\test_results.xml" --test-launcher-summary-output="e:\b\swarm_slave\w\iodoi7pd\output.json" --user-data-dir="C:\Users\CHROME~2\AppData\Local\Temp\scoped_dir6552_8905\d6552_10680" --disable-offline-auto-reload --no-first-run --no-default-browser-check --enable-logging=stderr --disable-default-apps --wm-window-animations-disabled --disable-component-update --test-type=browser --force-color-profile=srgb --disable-zero-browsers-open-for-tests --ipc-connection-timeout=30 --allow-file-access-from-files --dom-automation --log-gpu-control-list-decisions --disable-backgrounding-occluded-windows --disable-gl-drawing-for-tests --override-use-software-gl-for-tests --force-color-profile=srgb --disable-compositor-ukm-for-tests --enable-features=TestFeatureForBrowserTest1 --disable-features=NetworkPrediction,SpeculativePreconnect,TestFeatureForBrowserTest2 --disable-gpu-process-for-dx12-vulkan-info-collection --flag-switches-begin --flag-switches-end --file-url-path-alias="/gen=e:\b\swarm_slave\w\ir\out\Release_x64\gen" --restore-last-session about:blank
Received fatal exception EXCEPTION_ACCESS_VIOLATION
Backtrace:
	std::_Tree<std::_Tset_traits<syncer::syncable::Id,std::less<syncer::syncable::Id>,std::allocator<syncer::syncable::Id>,0> >::_Lbound<syncer::syncable::Id> [0x000000013FEA4A6A+10]
	std::_Tree<std::_Tset_traits<syncer::syncable::Id,std::less<syncer::syncable::Id>,std::allocator<syncer::syncable::Id>,0> >::find [0x000000013FE8461A+26]
	ui::ReplaceTemplateExpressions [0x0000000142C636F0+640]
	ui::I18nSourceStream::FilterData [0x0000000142C64523+307]
	net::FilterSourceStream::DoFilterData [0x0000000142A96D55+85]
	net::FilterSourceStream::DoLoop [0x0000000142A96836+54]
	net::FilterSourceStream::OnIOComplete [0x0000000142A96DCE+30]
	net::URLRequestJob::ReadRawDataComplete [0x0000000142B0971F+63]
	base::internal::PostTaskAndReplyImpl::PostTaskAndReply [0x00000001437A859B+779]
	base::internal::PostTaskAndReplyImpl::PostTaskAndReply [0x00000001437A8654+964]
	base::debug::TaskAnnotator::RunTask [0x000000014379352C+300]
	base::MessageLoop::RunTask [0x00000001428010A7+583]
	base::MessageLoop::DoWork [0x00000001428015F8+408]
	base::MessagePumpForIO::DoRunLoop [0x000000014280317A+330]
	base::MessagePumpWin::Run [0x000000014280249E+78]
	base::RunLoop::Run [0x0000000142823781+49]
	content::BrowserProcessSubThread::IOThreadRun [0x00000001416DFFC4+36]
	base::Thread::ThreadMain [0x0000000142846D70+384]
	base::PlatformThread::GetCurrentThreadPriority [0x0000000142845724+388]
	BaseThreadInitThunk [0x00000000772A59CD+13]
	RtlUserThreadStart [0x00000000774DB891+33]
[275/990] StartupBrowserCreatorWelcomeBackTest.WelcomeBackStandardNoPolicy (CRASHED)

Note that this should be fixed as part of this related bug: 863318