Example affected jobs:

https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Trusty/builds/28956

https://chromium-swarm.appspot.com/task?id=374f894d3864d310&refresh=10&show_raw=1

05:14:45.205 8642 "xdpyinfo -display :99" took 0.14s
05:14:45.211 8642 "xdpyinfo -display :100" took 0.01s
05:14:45.211 8642 Overriding TMPDIR to "/tmp" for Xvfb, was: /b/s/w/it__SVZ6
05:14:45.211 8642 Starting Xvfb with display ":100".
05:14:45.250 8642 "xdpyinfo" took 0.03s
05:14:45.250 8642 Successfully started Xvfb with display ":100".
05:14:45.250 8642 Checking system dependencies ...
05:14:45.251 8642 DISPLAY = :100
05:14:45.369 8642 "/b/s/w/ir/out/Release/content_shell --check-layout-test-sys-deps" took 0.12s
05:14:45.369 8642 System dependencies check failed.
05:14:45.369 8642 To override, invoke with --nocheck-sys-deps
05:14:45.369 8642 
05:14:45.369 8642 Xlib:  extension "RANDR" missing on display ":100".
[8664:8703:0712/051445.333987:14625043647:ERROR:devtools_http_handler.cc(786)] 
DevTools listening on 127.0.0.1:48271

[8701:8701:0712/051445.362153:14625071812:ERROR:sandbox_linux.cc(344)] InitializeSandbox() called with multiple threads in process gpu-process.
X Error of failed request:  BadWindow (invalid Window parameter)
  Major opcode of failed request:  20 (X_GetProperty)
  Resource id in failed request:  0x400006
  Serial number of failed request:  216
  Current serial number in output stream:  216
05:14:45.369 8642 
05:14:45.369 8642 For complete build requirements, please see:
05:14:45.369 8642 https://chromium.googlesource.com/chromium/src/+/master/docs/linux_build_instructions.md
05:14:45.373 8642 
05:14:45.374 8642 Testing completed, Exit status: 252