Fix and re-enable mojo_core_unittests on CFI bots

Adapts dynamic mojo_core library thunking to use base::ProtectedMemory
for function table storage and exempts the calls from cfi-icall checks.

Bug: 850464
Change-Id: I8e7d8dbcb4d312e089bdee830e89ed973ae5727e
Reviewed-on: https://chromium-review.googlesource.com/1105301
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568483}