Cleanup dependencies on MessageLoopForIO::current() in /net/cert_net

This was extracted from https://chromium-review.googlesource.com/c/chromium/src/+/957760/17

MessageLoopForIO::current() will soon stop returning a raw
MessageLoopForIO*. In preparation for this, fix old code to
use modern APIs instead of a direct MessageLoopForIO*.

This CL was uploaded by git cl split.

R=rsleevi@chromium.org

Bug: 825327
Change-Id: I317b493f88da3bcede2171e41b67feb96f44bde2
Reviewed-on: https://chromium-review.googlesource.com/1005887
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549851}
1 file changed