[gn] Enable stricter build flags

src/wasm/wasm-module-builder.cc

Index: src/wasm/wasm-module-builder.cc
diff --git a/src/wasm/wasm-module-builder.cc b/src/wasm/wasm-module-builder.cc
index 6b7f9104ac47abd4e8391079f4804b7a0835e537..6380b5d1550b00561344f5dc467fc8801c5a77e9 100644
--- a/src/wasm/wasm-module-builder.cc
+++ b/src/wasm/wasm-module-builder.cc
@@ -367,7 +367,7 @@ void WasmModuleBuilder::WriteTo(ZoneBuffer& buffer) const {
     buffer.write_size(functions_.size());
     for (auto function : functions_) {
       function->WriteSignature(buffer);
-      exports += function->exported_names_.size();
+      exports += static_cast<uint32_t>(function->exported_names_.size());
       if (function->name_.size() > 0) has_names = true;
     }
     FixupSection(buffer, start);