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-Commit-Position: refs/heads/master@{#14508}

webrtc/base/buffer.h

@@ -349,7 +349,7 @@ class BufferT {
   // Called when *this has been moved from. Conceptually it's a no-op, but we
   // can mutate the state slightly to help subsequent sanity checks catch bugs.
   void OnMovedFrom() {
-#ifdef NDEBUG
+#if RTC_DCHECK_IS_ON
     // Make *this consistent and empty. Shouldn't be necessary, but better safe
     // than sorry.
     size_ = 0;