Avoid calling RtpFrameObject::GetCodecHeader twice in the RtpFrameReferenceFinder.

This is good for a few reasons:
 - We avoid grabing a lock twice.
 - We avoid an unnecessary copy.
 - We avoid a race where a packet could potentially be removed before the
   second time we call GetCodecHeader.


Review-Url: https://codereview.webrtc.org/3009833002
Cr-Original-Commit-Position: refs/heads/master@{#19635}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4c14009e83bd147b440a785d7fb492da3355576c
2 files changed