Reland "Refactor rate profile update."

This is a reland of b6cdfdc165d76d86a67d829e0ccec50c36106e73

Original change's description:
> Refactor rate profile update.
> 
> RateProfile::frame_num specifies frame at which this rate profile
> should be applied.
> 
> Bug: none
> Change-Id: I003ee43f44299a49d83f547558284817bfaeacc0
> Reviewed-on: https://webrtc-review.googlesource.com/c/115242
> Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
> Reviewed-by: Seth Hampson <shampson@webrtc.org>
> Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#26080}

Bug: none
Change-Id: I2604878d0bbee0f2182ad74e3cc29546310b76f3
Reviewed-on: https://webrtc-review.googlesource.com/c/115401
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26145}