Avoid splitting functionality in two interface calls and passing around
a void* between them.

R=kschimpf@chromium.org

Change-Id: I0e12ff0295852ce5c36f7c3fb3b2c51dc0f2677e
Reviewed-on: https://chromium-review.googlesource.com/702484Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48335}