I don't think the culprit identified is actually the cause of the flakiness.

lethalantidote@: Could you PTAL and confirm?

Example failure:

[1:1:0912/175026.597432:FATAL:memory_dump_manager.cc(251)] Check failed: (*mdp_iter)->task_runner && (*mdp_iter)->task_runner->RunsTasksInCurrentSequence(). MemoryDumpProvider "ContextProviderCommandBuffer" attempted to unregister itself in a racy way. Please file a crbug.
#0 0x7f5df4ba3a5d base::debug::StackTrace::StackTrace()
#1 0x7f5df48b560c base::debug::StackTrace::StackTrace()
#2 0x7f5df491f61d logging::LogMessage::~LogMessage()
#3 0x7f5df4afcbda base::trace_event::MemoryDumpManager::UnregisterDumpProviderInternal()
#4 0x7f5df4afc16f base::trace_event::MemoryDumpManager::UnregisterDumpProvider()
#5 0x7f5dec274149 ws::ContextProviderCommandBuffer::~ContextProviderCommandBuffer()
#6 0x7f5dec274af9 ws::ContextProviderCommandBuffer::~ContextProviderCommandBuffer()
#7 0x7f5dec27c89b base::RefCountedThreadSafe<>::DeleteInternal<>()
#8 0x7f5dec27c865 base::DefaultRefCountedThreadSafeTraits<>::Destruct()
#9 0x7f5dec27bf6c base::RefCountedThreadSafe<>::Release()
#10 0x7f5dec274dd9 ws::ContextProviderCommandBuffer::Release()

The comment above the DCHECK says

    // If you hit this DCHECK, your dump provider has a bug.
    // Unregistration of a MemoryDumpProvider is safe only if:
    // - The MDP has specified a sequenced task runner affinity AND the
    //   unregistration happens on the same task runner. So that the MDP cannot
    //   unregister and be in the middle of a OnMemoryDump() at the same time.
    // - The MDP has NOT specified a task runner affinity and its ownership is
    //   transferred via UnregisterAndDeleteDumpProviderSoon().
    // In all the other cases, it is not possible to guarantee that the
    // unregistration will not race with OnMemoryDump() calls.

I can't tell from the stack trace where this dump provider is. +primiano who added that DCHECK :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cd5317e7c3e26c1eab245f06643a0098ea6e4bdc

commit cd5317e7c3e26c1eab245f06643a0098ea6e4bdc
Author: Darren Shen <shend@chromium.org>
Date: Fri Sep 14 04:16:06 2018

[sheriff] Disable flaky MediaEngagementAutoplayBrowserTest test.

Timing out from crashing on Linux.

TBR=beccahughes@chromium.org

Bug:  883706 
Change-Id: I4c618b6c4ad88a895ba53cd177106487fec8eba7
Reviewed-on: https://chromium-review.googlesource.com/1226475
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591287}
[modify] https://crrev.com/cd5317e7c3e26c1eab245f06643a0098ea6e4bdc/chrome/browser/media/media_engagement_autoplay_browsertest.cc

that's some threading issue with ws::ContextProviderCommandBuffer, which calls unregister on a different task runner than the one passed here:
https://cs.chromium.org/chromium/src/services/ws/public/cpp/gpu/context_provider_command_buffer.cc?sq=package:chromium&g=0&l=89

+sadrul to route this bug

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/47a3f38c37bc545b01c1f8eb90322ff057320fac

commit 47a3f38c37bc545b01c1f8eb90322ff057320fac
Author: James Cook <jamescook@chromium.org>
Date: Thu Sep 20 19:21:11 2018

Re-enable MediaEngagementAutoplayBrowserTest.UsePreloadedData_Allowed

It was disabled on all Linux platforms due to failures on Chrome OS
in SingleProcessMash browser_tests.

Instead, disable all MediaEngagement browser_tests via filter file
for SingleProcessMash. There's a fix in-flight for renderer
embedding in this mode, so I expect to re-enable them shortly.

Bug:  884589 ,  883706 , 881574
Change-Id: If6257ea160c7ba83c9bb322941001ce5ab1cc1b7
Reviewed-on: https://chromium-review.googlesource.com/1236836
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592897}
[modify] https://crrev.com/47a3f38c37bc545b01c1f8eb90322ff057320fac/chrome/browser/media/media_engagement_autoplay_browsertest.cc
[modify] https://crrev.com/47a3f38c37bc545b01c1f8eb90322ff057320fac/testing/buildbot/filters/chromeos.single_process_mash.browser_tests.filter