Disable V8 postmortem debugging on Windows

It is not supported by V8.

common.gypi

@@ -8,7 +8,13 @@
     'library%': 'static_library',    # allow override to 'shared_library' for DLL/.so builds
     'component%': 'static_library',  # NB. these names match with what V8 expects
     'msvs_multi_core_compile': '0',  # we do enable multicore compiles, but not using the V8 way
-    'v8_postmortem_support': 'true', # V8's postmortem metadata
+
+    # Enable V8's post-mortem debugging only on unix flavors.
+    'conditions': [
+      ['OS != "win"', {
+        'v8_postmortem_support': 'true'
+      }]
+    ],
   },
 
   'target_defaults': {