DecoderDatabase::IsComfortNoise: Improved efficiency.
DecoderDatabase::IsComfortNoise: Improved efficiency.

Changed DecoderDatabase::IsComfortNoise to do one, rather than four,
lookups of the rtp payload type. IsComfortNoise is called more frequently
since CNG was changed to not be an AudioDecoder.

BUG=606765

Review URL: https://codereview.webrtc.org/1923763003

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#12533}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6353723b029de581b1d0393ddf49c79f34b8f37c

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc