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This is needed to avoid race conditions with setting a cookie and making a request right away.
Currently it works when cookies and request handling are dispatched on the same thread and
associated mojo interfaces are used. However this will stop working if non-associated interfaces
are used, which will be the case when the cookie and loading interfaces live in different processes.
More background here:
https://docs.google.com/a/chromium.org/document/d/1eg1ohplfFGrlz5gFNgbJflRdIljPqCj58p4Zfb81QGM

We don't anticipate this having a perceptible perf impact, but added UMA stats to confirm. Cookie
getters are already sync, and this new sync call replies right when the IPC is dispatched (as
opposed to when the cookie database is updated).

BUG=721395

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: Icb7afe358a9fa969cd6fed1c2c5730fa9a18a2ec
Reviewed-on: https://chromium-review.googlesource.com/742345
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513123}