DEBUG_MODE was originally provided to allow code of the form:

  if (DEBUG_MODE) { ... }

rather than preprocessor conditional (i.e. #ifdef _DEBUG), while
still ensuring that the condition was static at compile-time, allowing
the affected code to be trivially optimized-out.

The function-like macro DCHECK_IS_ON(), added to simplify conditional
compilation of DCHECK-related code when DCHECK_ALWAYS_ON was added, can
now be used safely for both preprocessor and C++ conditionals in place of
DEBUG_MODE.

Review-Url: https://codereview.chromium.org/2731823002
Cr-Commit-Position: refs/heads/master@{#456519}