Disable DCHECK for external reference address uniqueness on Windows

The memcpy and memmove externals can end up at the same address; see bug for
details.

BUG=chromium:726896

Review-Url: https://codereview.chromium.org/2906193002
Cr-Commit-Position: refs/heads/master@{#45545}

src/snapshot/serializer-common.cc

@@ -24,7 +24,11 @@ ExternalReferenceEncoder::ExternalReferenceEncoder(Isolate* isolate) {
     Address addr = table->address(i);
     // Ignore duplicate API references.
     if (table->is_api_reference(i) && !map_->Get(addr).IsNothing()) continue;
+#ifndef V8_OS_WIN
+    // TODO(yangguo): On Windows memcpy and memmove can end up at the same
+    // address due to ICF. See http://crbug.com/726896.
     DCHECK(map_->Get(addr).IsNothing());
+#endif
     map_->Set(addr, i);
     DCHECK(map_->Get(addr).IsJust());
   }