StreamSynchronizationTest: rename and make some variables const.
StreamSynchronizationTest: rename and make some variables const.

Bug: none
Change-Id: I5c452b0d2f58b2821db31b19506de2ba73480748
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168125
Reviewed-by: Rasmus Brandt <9545de3a34b925a736ae2b5ee593f3033f743a28@webrtc.org>
Commit-Queue: Åsa Persson <5ff7466874d1fe71d5fe7d79a8a23f5ef50662da@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30537}

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