Also adding kHz factory function for Frequency class. Bug: webrtc:9883 Change-Id: Ide44910d50eb9616de2bb0c66b8c62493d2be92e Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/167725 Commit-Queue: Sebastian Jansson <srte@webrtc.org> Reviewed-by: Ali Tofigh <alito@webrtc.org> Cr-Commit-Position: refs/heads/master@{#30416}