Allow repeat frames a first frame in EncoderSpeedController.

The purpose of special casing repeat-frame is that since they will be
very similar to the last frame, they will be artificially "easy" to
encode.

However, if an encoder is reconfigured mid stream it might start with a
repeat-frame, in which case it will be in effect a brand new frame. In
the same way, keyframes should also treat repeat-frames as regular ones.

Bug: webrtc:443906251
Change-Id: Ida738d498e0c2b374a271cb84fc7a4dbb2433e4a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/488381
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48179}