Test RTC_DCHECK_IS_ON instead of checking DCHECK_ALWAYS_ON everywhere

The former is always defined (by webrtc/base/checks.h) to either 0 or
1, whereas the latter isn't necessarily defined.

NOTRY=true
BUG=webrtc:6451

Review-Url: https://codereview.webrtc.org/2384693002
Cr-Original-Commit-Position: refs/heads/master@{#14474}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: ab0b929321d37669165d5795268fa10a8c97ec5b
15 files changed