Fix NarrowingCompoundAssignment warning
Fix NarrowingCompoundAssignment warning

This ErrorProne warning was enabled in
http://crrev.com/96c7ab0153ae97a8d8e05949f36cd7bb8eedbf1d
https://webrtc-review.googlesource.com/60849

Bug: None
Change-Id: I5e622f84925ee96e7743d2c08d17fcdb4c4a0f55
Reviewed-on: https://webrtc-review.googlesource.com/60940
Reviewed-by: Sami Kalliomäki <b9debeb8943b89663e00c95863751401cba91c9a@webrtc.org>
Commit-Queue: Oleh Prypin <5baa3e04f9bd6ba7a2a4b56625a3c84f19237cdb@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#22362}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc