Optional: Use nullopt and implicit construction in /rtc_base/rate_statistics.cc
Optional: Use nullopt and implicit construction in /rtc_base/rate_statistics.cc

Changes places where we explicitly construct an Optional to instead use
nullopt or the requisite value type only.

This CL was uploaded by git cl split.

R=3ab8c90c99d6dd7822e0fad803ab0211dd287ae5@webrtc.org

Bug: None
Change-Id: I50d25d6174486928963c2e98455587a8a9f0bee6
Reviewed-on: https://webrtc-review.googlesource.com/23616
Reviewed-by: Erik Språng <3ab8c90c99d6dd7822e0fad803ab0211dd287ae5@webrtc.org>
Commit-Queue: Oskar Sundbom <5d38decb5d5e61546cd18e5e1cbaa7a6fbf39c74@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#20930}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc