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.

This is a re-land of https://codereview.webrtc.org/2384693002, which
broke Chromium. We re-land without changing this CL at all, because
the thing that needed fixing was in Chromium:
https://codereview.chromium.org/2384263004.

NOTRY=true
TBR=ossu@webrtc.org
BUG=webrtc:6451

Review-Url: https://codereview.webrtc.org/2389943003
Cr-Original-Commit-Position: refs/heads/master@{#14508}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5377bc77cc9ba2a61001e38f40e6d253aba961aa
15 files changed