sdp: use const references to fields
sdp: use const references to fields

BUG=webrtc:11718

Change-Id: I60bdfcdadef665f6044525046ec25717102365d4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178363
Reviewed-by: Tommi <74a97831ec830b6e3846c6faa50560947ae01c68@webrtc.org>
Commit-Queue: Tommi <74a97831ec830b6e3846c6faa50560947ae01c68@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#31580}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc