Don't DCHECK in release builds

NDEBUG was defined for Release builds on Windows, but not on POSIX.

Bug: chromium:856878
Change-Id: I817670c0aa2389da61741a031421f622a1694a16
Reviewed-on: https://gn-review.googlesource.com/1860
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index 26e23ad..a0796fb 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -172,7 +172,7 @@
     if options.debug:
       cflags.extend(['-O0', '-g'])
     else:
-      cflags.extend(['-O3', '-flto'])
+      cflags.extend(['-O3', '-flto', '-DNDEBUG'])
       ldflags.append('-Wl,-S' if is_mac else '-Wl,-strip-all')
       ldflags.append('-flto')