Resolve console output in postMessage tests with site-per-process.
Regular tests do not dump console output anymore, and special same-site test does it.
BUG=
614413
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation
Committed:
https://crrev.com/6b50e7bb8127b82d23787cb4ee1975a6e3758c82
Cr-Commit-Position: refs/heads/master@{#396055}