Change DECHECKs to CHECKs in `base::RingBuffer`.

Better safe than sorry!

In the process, do some micro-optimization to make the CHECKs as cheap as
possible.

Bug: 817982
Change-Id: I2c45d7e773199a12ce4ae34b4a750ac2b66cb7c2
Reviewed-on: https://chromium-review.googlesource.com/1134486
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574656}
1 file changed