Introduce a gyp variable to control whether or not slow dchecks are on

build/toolchain.gypi

Index: build/toolchain.gypi
diff --git a/build/toolchain.gypi b/build/toolchain.gypi
index ac10065601a0c1e426e69a2fcde7307cc8b9648c..8227fbf8e03e6808d2cf39b12a2b1aeb04cf0a90 100644
--- a/build/toolchain.gypi
+++ b/build/toolchain.gypi
@@ -883,6 +883,9 @@
             'LinkIncremental': '2',
           },
         },
+        'variables': {
+          'v8_enable_slow_dchecks%': 1,
+        },
         'conditions': [
           ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="netbsd" or \
             OS=="qnx"', {
@@ -902,9 +905,11 @@
                'GCC_OPTIMIZATION_LEVEL': '0',  # -O0
             },
           }],
-        ],
-        'defines': [
-          'ENABLE_SLOW_DCHECKS',
+          ['v8_enable_slow_dchecks==1', {
+            'defines': [
+              'ENABLE_SLOW_DCHECKS',
+            ],
+          }],
         ],
       },  # DebugBase0
       # Abstract configuration for v8_optimized_debug == 1.
@@ -930,9 +935,9 @@
             'LinkIncremental': '2',
           },
         },
-        'defines': [
-          'ENABLE_SLOW_DCHECKS',
-        ],
+        'variables': {
+          'v8_enable_slow_dchecks%': 1,
+        },
         'conditions': [
           ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="netbsd" or \
             OS=="qnx"', {
@@ -954,6 +959,11 @@
                'GCC_STRICT_ALIASING': 'YES',
             },
           }],
+          ['v8_enable_slow_dchecks==1', {
+            'defines': [
+              'ENABLE_SLOW_DCHECKS',
+            ],
+          }],
         ],
       },  # DebugBase1
       # Abstract configuration for v8_optimized_debug == 2.
@@ -981,6 +991,9 @@
             'EnableCOMDATFolding': '2',
           },
         },
+        'variables': {
+          'v8_enable_slow_dchecks%': 0,
+        },
         'conditions': [
           ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="netbsd" or \
             OS=="qnx"', {
@@ -1010,6 +1023,11 @@
               'GCC_STRICT_ALIASING': 'YES',
             },
           }],
+          ['v8_enable_slow_dchecks==1', {
+            'defines': [
+              'ENABLE_SLOW_DCHECKS',
+            ],
+          }],
         ],
       },  # DebugBase2
       # Common settings for the Debug configuration.
@@ -1064,6 +1082,9 @@
         ],
       },  # Debug
       'Release': {
+        'variables': {
+          'v8_enable_slow_dchecks%': 0,
+        },
         'conditions': [
           ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="netbsd"', {
             'cflags!': [
@@ -1130,6 +1151,11 @@
               },
             },
           }],  # OS=="win"
+          ['v8_enable_slow_dchecks==1', {
+            'defines': [
+              'ENABLE_SLOW_DCHECKS',
+            ],
+          }],
         ],  # conditions
       },  # Release
     },  # configurations