SystemTimeNanos() on Mac: DCHECK that multiplication doesn't overflow
SystemTimeNanos() on Mac: DCHECK that multiplication doesn't overflow

Bug: webrtc:6149
Change-Id: I1a58b60838d924e40e89126110cc19e16c953e80
Reviewed-on: https://webrtc-review.googlesource.com/56701
Commit-Queue: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Reviewed-by: Niels Moller <0057859e47d61d90529d0f66d9ef692cefedb46a@webrtc.org>
Reviewed-by: Björn Terelius <efeae16329b3cb4ac5daf80d4bfda3344c027db3@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#22194}
TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc