We had dangling pointers by storing a raw pointer and then discarding
the unique_ptr holding it alive, and we had lots of redundant
information there.
This CL refactors the interface to take a format string and a variable
number of argument.

R=titzer@chromium.org

Change-Id: I8eb6ccd19d307e2477c97a3e5e7f537b5671a891
Reviewed-on: https://chromium-review.googlesource.com/690196
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48223}