$ strace -e trace=process,signal -f -o strace.log -ff testing/xvfb.py blink/tools/run_layout_tests.py fast/history/form-submit-in-frame-via-onclick.html
Additional test environment:
    CHROME_DEVEL_SANDBOX=/opt/chromium/chrome_sandbox
    LANG=en_US.UTF-8
Command: /usr/bin/python blink/tools/run_layout_tests.py fast/history/form-submit-in-frame-via-onclick.html

Using port 'linux-trusty'
Test configuration: <trusty, x86_64, release>
View the test results at file:///usr/local/google/home/dcheng/src/chrome/src/out/Release/layout-test-results/results.html
Using random order with seed: 1506820065
Baseline search path: linux -> win -> generic
Using Release build
Pixel tests enabled
Regular timeout: 6000, slow test timeout: 30000
Command line: /usr/local/google/home/dcheng/src/chrome/src/out/Release/content_shell --run-layout-test --ignore-certificate-errors-spki-list=Nxvaj3+bY3oVrTc+Jp7m3E3sB1n3lXtnMDCyBsqEXiY= --enable-crash-reporter --crash-dumps-dir=/usr/local/google/home/dcheng/src/chrome/src/out/Release/crash-dumps -

Found 1 test; running 1, skipping 0.
Running 1 content_shell.                                                                      

[1/1] fast/history/form-s...me-via-onclick.html failed unexpectedly (text diff by spaces and tabs only)
                              
0 tests ran as expected, 1 didn't:

packet_write_wait: Connection to UNKNOWN port 0: Broken pipe

Logging back in and looking at strace.log, one of the logs has something interesting:
kill(4294967295, SIGTERM)               = 0
rt_sigprocmask(SIG_BLOCK, 0x7ffcacf65290, [?], 8) = 0
rt_sigaction(SIGCHLD, NULL, {...}, 8)   = 0 
rt_sigprocmask(SIG_SETMASK, 0x7ffcacf65310, NULL, 8) = 0
kill(4294967295, SIGKILL

This process appears to be related to dbus:
execve("/usr/bin/dbus-launch", ["dbus-launch", "--autolaunch", "75781416a1e7313e9a0aa75f581cd265", "--binary-syntax", "--close-stderr"], [/* 36 vars */]) = 0

I think this is coming from xdg-open, which is somehow being invoked by the layout test runner to show failing tests at the end of the run.

Quick note here is that run-webkit-tests doesn't try to show the results in the browser if the --no-show-results flag is passed. Default is to try to show the results in the browser when there are failures.

run-webkit-tests doesn't directly invoke xdg-open, but instead opens it through the Python webbrowser module at:
https://cs.chromium.org/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/common/system/user.py?=114

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/22be5ece562b55c77aeaa6a5b3e13c8208f4c378

commit 22be5ece562b55c77aeaa6a5b3e13c8208f4c378
Author: Daniel Cheng <dcheng@chromium.org>
Date: Tue Oct 03 07:02:12 2017

Don't try to show layout test results inside testing/xvfb.py

dbus often isn't running inside the testing environment. Showing the
test results often results in an autolaunched dbus, which has the
unfortunate habit of sending SIGTERM and SIGKILL to all processes when
testing/xvfb.py exits.

Bug:  770493 
Change-Id: I49a205976f07547062ee8aa73749cfc117b3e103
Reviewed-on: https://chromium-review.googlesource.com/696021
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505971}
[modify] https://crrev.com/22be5ece562b55c77aeaa6a5b3e13c8208f4c378/third_party/WebKit/Tools/Scripts/webkitpy/common/system/user.py