Also take the opportunity to declare factory_ const.
(Bug reference is where I noticed the possibility; it is unlikely
to fix the bug.)
Bug: chromium:1205343
Change-Id: I6078f170cf68d94314ee184bdfd2dc6f4ffc1e71
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/217385
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33918}
9582fa47db