GN: Allow users to enable DCHECK on Release builds.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index c57ba3f317c5da6d4d83040b4e49b53db97d7417..5f93dd9dc2fd78d4012c1641951973c5904ed89c 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -19,6 +19,9 @@ declare_args() {
   # TODO(sebmarchand): Update this comment once this flag guarantee that
   #     there's no build metadata in the build artifacts.
   dont_embed_build_metadata = false
+
+  # Set to true to enable dcheck in Release builds.
+  dcheck_always_on = false
 }
 
 # TODO(brettw) Most of these should be removed. Instead of global feature
@@ -74,6 +77,9 @@ config("feature_flags") {
   if (dont_embed_build_metadata) {
     defines += [ "DONT_EMBED_BUILD_METADATA" ]
   }
+  if (dcheck_always_on) {
+    defines += [ "DCHECK_ALWAYS_ON=1" ]
+  }
   if (use_udev) {
     # TODO(brettw) should probably be "=1".
     defines += [ "USE_UDEV" ]