The relevant log is

18:02:56.472 27393 worker/2 fast/workers/worker-multi-startup.html output stderr lines:
18:02:56.472 27393   
18:02:56.472 27393   DevTools listening on ws://127.0.0.1:42267/devtools/browser/37eedf89-42f9-4bc9-9a6f-db84cf190cf2
18:02:56.472 27393   Xlib:  extension "RANDR" missing on display ":100".
18:02:56.475 14739 [5/5] fast/workers/worker-multi-startup.html failed unexpectedly (test timed out)

It is approximately the same for all 3 builders above.

I am going to disable the test.

nhiroki@ could you look into this (or reassign) please?

Submitting the CL to ignore the test timeouts on Linux Debug

https://chromium-review.googlesource.com/c/chromium/src/+/628168

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0265fd83666006369852487d3abe0f5310806194

commit 0265fd83666006369852487d3abe0f5310806194
Author: Vitalii Iarko <vitaliii@chromium.org>
Date: Wed Aug 23 10:55:01 2017

Ignore timeouts of fast/workers/worker-multi-startup.html because flaky.

They are ignored only on Linux in Debug.

TBR=nhiroki@chromium.org

Bug:  758109 
Change-Id: I25570e3453fe9b0d7dfaa6e4bc39207aa34301e2
Reviewed-on: https://chromium-review.googlesource.com/628168
Reviewed-by: vitaliii <vitaliii@chromium.org>
Commit-Queue: vitaliii <vitaliii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496651}
[modify] https://crrev.com/0265fd83666006369852487d3abe0f5310806194/third_party/WebKit/LayoutTests/TestExpectations

See the flakiness dashboard here:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_tests&tests=fast%2Fworkers%2Fworker-multi-startup.html

The test started failing at:
https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Trusty%20(dbg)/builds/4250

I suspect this is caused by peria@'s change:
(Reland)[Bindings] Create and use V8 context snapshots.
https://chromium.googlesource.com/chromium/src/+/f2f50dec16d633a28d799ee5c3223159444e9e46

I suspect this is somewhat expected by the snapshot change made isolate creation slower in order to make context creation faster, and we create one isolate per worker thread.

But I'm not sure why WebKit Linux Trusty (dbg) is seemingly the only bot that this fails on, and it fails on each bot job (it does TIMEOUT at least once).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d1273287aac4f594e35b81de768f15eb6afafac8

commit d1273287aac4f594e35b81de768f15eb6afafac8
Author: Matt Falkenhagen <falken@chromium.org>
Date: Fri Oct 06 06:29:46 2017

Gardening: fast/workers/worker-multi-startup.html now passes

The test is passing. This was disabled for slowness after the V8 Context Snapshotting change
https://chromium.googlesource.com/chromium/src/+/f2f50dec16d633a28d799ee5c3223159444e9e46,
but later the regression on worker startup performance by snapshotting was fixed.

Bug:  758109 
Change-Id: I034b25f8b399a67485658d85b29c7a67378f5803
TBR: nhiroki
Reviewed-on: https://chromium-review.googlesource.com/704416
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506996}
[modify] https://crrev.com/d1273287aac4f594e35b81de768f15eb6afafac8/third_party/WebKit/LayoutTests/TestExpectations