Improved readability and DCHECKing in AudioVector::[]
Improved readability and DCHECKing in AudioVector::[]

This is a follow-up to https://codereview.webrtc.org/2700633003, where
post-commit comments suggested these changes.

BUG=webrtc:7159

Review-Url: https://codereview.webrtc.org/2706263002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16771}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5650a7d1c41a264a0da15c3bb17d2c14335fcc27

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk