Reland "Default to dcheck_always_on=true in is_syzyasan builds."

This is a reland of 597f4baeedd9daaf67c4d229d48a326626a914f3

This is safe to re-land now as V8's DCHECK implementation has been
diverted into the base DCHECK implementation in SyzyASAN builds.

Original change's description:
> Default to dcheck_always_on=true in is_syzyasan builds.
> 
> This causes all SyzyASAN builds to enable DCHECKs. Previous CLs have
> landed to make the this combination result in binaries which default to
> evaluating DCHECKs without treating them as fatal, unless configured to
> at run-time.
> 
> Bug: 596231
> Change-Id: I15f23ba3af63c572691e9ddf96cd82780b6bac60
> Reviewed-on: https://chromium-review.googlesource.com/644358
> Commit-Queue: Wez <wez@chromium.org>
> Reviewed-by: Scott Graham <scottmg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#498735}

TBR: scottmg@chromium.org
Bug: 596231
Change-Id: Ie7530419c30e9c9f4c50ff8d8b23db7476d30cb0
Reviewed-on: https://chromium-review.googlesource.com/677004
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Robert Shield <robertshield@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#503444}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 71c72ca703fcb9fb0754c2bb0e1403f7982250ea

config/dcheck_always_on.gni

@@ -2,7 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/sanitizers/sanitizers.gni")
+
 declare_args() {
   # Set to true to enable dcheck in Release builds.
-  dcheck_always_on = false
+  # is_syzyasan defaults to logging/non-fatal DCHECKs - see crbug.com/596231.
+  dcheck_always_on = is_syzyasan
 }