Failed a couple of times without an obvious culprit:

https://luci-milo.appspot.com/buildbot/chromium.webkit/WebKit%20Linux%20Trusty/34768
https://luci-milo.appspot.com/buildbot/chromium.webkit/WebKit%20Linux%20Trusty/34767

3:09:03.994 7161 worker/1 virtual/mojo-loading/http/tests/security/contentSecurityPolicy/multiple-report-policies.php output stderr lines:
13:09:03.994 7161   [7170:7296:1024/130858.000169:28140423654:ERROR:service_manager.cc(157)] Connection InterfaceProviderSpec prevented service: content_renderer from binding interface: blink::mojom::ReportingServiceProxy exposed by: content_browser
13:09:03.996 7018 [5147/10691] virtual/mojo-loading/http/tests/security/contentSecurityPolicy/multiple-report-policies.php failed unexpectedly (test timed out)
13:09:03.994 7161 worker/1 killing primary driver
13:09:03.994 7161 worker/1 killing secondary driver
13:09:03.994 7161 worker/1 virtual/mojo-loading/http/tests/security/contentSecurityPolicy/multiple-report-policies.php failed:
13:09:03.994 7161 worker/1  test timed out