Actually, this didn't repro for me locally, and I noticed that the flakiness dashboard shows this failing on a couple of other bots:

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_tests%20(with%20patch)&tests=http%2Ftests%2Fsecurity%2Freferrer-policy-attribute-import-no-referrer.html

So maybe this isn't specific to --site-per-process and involves something else in those bot configs?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/96557bfa11c102628943935e7906cff4b3b04a74

commit 96557bfa11c102628943935e7906cff4b3b04a74
Author: alexmos <alexmos@chromium.org>
Date: Thu Jan 12 00:14:49 2017

Add expectations for three layout tests failing on Site Isolation bots.

This should hopefully get the bots back to green while individual bugs
are being investigated.

BUG= 680201 ,  661725 ,  680249 ,  680307 
NOTRY=true

Review-Url: https://codereview.chromium.org/2625233002
Cr-Commit-Position: refs/heads/master@{#443071}

[modify] https://crrev.com/96557bfa11c102628943935e7906cff4b3b04a74/third_party/WebKit/LayoutTests/FlagExpectations/site-per-process

Failing to reproduce locally. Is this test still failing?

I also can't reproduce locally (with --additional-drt-flag=--site-per-process and with --iterations=50).  Let me try to reenable this test in https://codereview.chromium.org/2814563002 and see what linux_site_isolation trybot says.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dbdb1d18c66296934a3ec0bd01b0f516030c8fd0

commit dbdb1d18c66296934a3ec0bd01b0f516030c8fd0
Author: lukasza <lukasza@chromium.org>
Date: Tue Apr 11 23:09:59 2017

OOPIFs: Enable h/t/s/referrer-policy-attribute-import-no-referrer.html test.

This test seemed to have mysteriously "healed" itself.

BUG= 680201 

Review-Url: https://codereview.chromium.org/2814563002
Cr-Commit-Position: refs/heads/master@{#463823}

[modify] https://crrev.com/dbdb1d18c66296934a3ec0bd01b0f516030c8fd0/third_party/WebKit/LayoutTests/FlagExpectations/site-per-process

Let's monitor the site isolation trybots and FYI bots for a while and close this bug if http/tests/security/referrer-policy-attribute-import-no-referrer.html keeps being green.

No repro - let's close the bug.