Style guide fixes for the hkdf class.
Style guide fixes for the hkdf class.

Bug: webrtc:9860
Change-Id: I762d175bbf2c240feb476bbf6d91a1a748d9bcbb
Reviewed-on: https://webrtc-review.googlesource.com/c/125125
Reviewed-by: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Reviewed-by: Qingsi Wang <1c4f50fe02fab79f81be17f4e2f574c199f33efd@webrtc.org>
Commit-Queue: Benjamin Wright <d97cb5a683a42395ddbd3524ae4315b2c4b93d14@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#26930}
TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc