Turn on DCHECKs and other debugging code if dcheck_always_on is 1

build/features.gypi

Index: build/features.gypi
diff --git a/build/features.gypi b/build/features.gypi
index 6cb0c90cd8ccd793e57f86b81fb7fd48628e5be6..c5b7c425b14aedf1630e57573cce3528c3ff96c2 100644
--- a/build/features.gypi
+++ b/build/features.gypi
@@ -62,6 +62,9 @@
     # Use external files for startup data blobs:
     # the JS builtins sources and the start snapshot.
     'v8_use_external_startup_data%': 0,
+
+    # Set to 1 to enable DCHECKs in release builds.
+    'dcheck_always_on%': 0,
   },
   'target_defaults': {
     'conditions': [
@@ -95,6 +98,9 @@
       ['v8_use_external_startup_data==1', {
         'defines': ['V8_USE_EXTERNAL_STARTUP_DATA',],
       }],
+      ['dcheck_always_on!=0', {
+        'defines': ['DEBUG',],
+      }],
     ],  # conditions
     'configurations': {
       'DebugBaseCommon': {