Example log: 03:17:31.099 38757 worker/7 fast/history/history-back-twice-with-subframes-assert.html output stderr lines:
03:17:31.099 38757   objc[38808]: Class MockCrApp is implemented in both /b/s/w/ir/out/Debug/libtest_runner.dylib and /b/s/w/ir/out/Debug/Content Shell.app/Contents/Frameworks/Content Shell Framework.framework/Content Shell Framework. One of the two will be used. Which one is undefined.
03:17:31.099 38757   objc[38809]: Class MockCrApp is implemented in both /b/s/w/ir/out/Debug/libtest_runner.dylib and /b/s/w/ir/out/Debug/Content Shell.app/Contents/Frameworks/Content Shell Framework.framework/Content Shell Framework. One of the two will be used. Which one is undefined.
03:17:31.099 38757   [38808:775:0102/031713.393161:WARNING:vt_video_decode_accelerator_mac.cc(178)] Failed to create VTDecompressionSession: Error Domain=NSOSStatusErrorDomain Code=-8973 "codecOpenErr" (-8973)
03:17:31.099 38757   [38808:775:0102/031713.393689:WARNING:vt_video_decode_accelerator_mac.cc(200)] Failed to create hardware VideoToolbox session
03:17:31.099 38757   [38808:775:0102/031713.429061:ERROR:vt_video_encode_accelerator_mac.cc(517)]  VTCompressionSessionCreate failed: -12908
03:17:31.102 36610 [14/17] fast/history/history-back-twice-with-subframes-assert.html failed unexpectedly (test timed out)

Tyoshino: assigning to you as you were the last one to touch that file.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ccba8d4045f73a818cf7d57e367e35382c767ba8

commit ccba8d4045f73a818cf7d57e367e35382c767ba8
Author: maxmorin <maxmorin@chromium.org>
Date: Tue Jan 02 14:14:26 2018

back-twice-with-subframes-assert.html flaky.

fast/history/history-back-twice-with-subframes-assert.html flakily
times out.

Bug:  798121 
Change-Id: Idfb693776a84ca1946ec0771fcd2fd2f71a80f10
Tbr: 
Reviewed-on: https://chromium-review.googlesource.com/846990
Reviewed-by: Max Morin <maxmorin@chromium.org>
Commit-Queue: Max Morin <maxmorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526471}
[modify] https://crrev.com/ccba8d4045f73a818cf7d57e367e35382c767ba8/third_party/WebKit/LayoutTests/TestExpectations

UI>Browser>History and UI>Browser>Navigation seem like the incorrect components for flaky webkit test. Please re-add if that's not the case.

Flaky tests are assigned to the component they're tested. In this case history navigation.

Blink>Layout is for page layout, not layout tests.

tentatively assigned to me

This test was introduced at Oct/19/2011.
https://chromium.googlesource.com/chromium/src/+/1a1b55c1bd24579445d40673e0d386904f8225a8

All failures in the record were due to timeout, but test looks it can finish quickly.

Let me just remove the flaky mark and see what happens after that. It this still fails, Navigation members will be better to take a closer look, I think.

Adding History and Navigation components.

fyi, it works fine on local machine, off course.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/be06f2f2a79904493976ebca714c33a4e06635ba

commit be06f2f2a79904493976ebca714c33a4e06635ba
Author: Takashi Toyoshima <toyoshim@chromium.org>
Date: Fri Mar 09 11:46:22 2018

Remove flakiness mark of history-back-twice-with-subframes-assert.html

Since it looks working fine on local machines, and related code base
may be changed day by day for loading and PlzNavigate efforts,
let me just re-enable the test to see what happens.

Bug:  798121 
Change-Id: I07e88b163818019ec340b3aff508191852b7d440
TBR: maxmorin@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/956830
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542083}
[modify] https://crrev.com/be06f2f2a79904493976ebca714c33a4e06635ba/third_party/WebKit/LayoutTests/TestExpectations

no failure in two days