Add build system infrastructure for ENABLE_EXTRA_CHECKS flag (not used yet)

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index b8cd11098f9ad21b3988465b4cc3770f251a7543..96055d22f60b20fdf22c7a83ddd9b116f7f2fdfc 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -70,6 +70,9 @@
 
     'v8_enable_disassembler%': 0,
 
+    # Enable extra checks in API functions and other strategic places.
+    'v8_enable_extra_checks%' 1,
+
     'v8_object_print%': 0,
 
     'v8_enable_gdbjit%': 0,
@@ -109,6 +112,9 @@
       ['v8_enable_disassembler==1', {
         'defines': ['ENABLE_DISASSEMBLER',],
       }],
+      ['v8_enable_extra_checks==1', {
+        'defines': ['ENABLE_EXTRA_CHECKS',],
+      }],
       ['v8_object_print==1', {
         'defines': ['OBJECT_PRINT',],
       }],