base: Change DCHECK_IS_ON to a macro DCHECK_IS_ON().
This ensures that if the header is not included, and a DCHECK is guarded
by this check, that the file will fail to compile instead of silently
compiling the DCHECK out.
For example:
#if DCHECK_IS_ON
DCHECK(SomeThing());
#endif
This example would be compiled out if DCHECK_IS_ON was not defined due
to not including the logging.h header.
Instead, this will fail to compile:
#if DCHECK_IS_ON()
DCHECK(SomeThing());
#endif
R=thakis@chromium.org
Review URL: https://codereview.chromium.org/
842523002
Cr-Commit-Position: refs/heads/master@{#310626}