[wasm] Canonicalize function signature indices for matching in indirect calls.

src/wasm/wasm-module-builder.h

Index: src/wasm/wasm-module-builder.h
diff --git a/src/wasm/wasm-module-builder.h b/src/wasm/wasm-module-builder.h
index ed08923d35c70434c743d6e91d99cc770b467998..4f463b01df7c55d8ec8be840af2d72931bb320b8 100644
--- a/src/wasm/wasm-module-builder.h
+++ b/src/wasm/wasm-module-builder.h
@@ -229,6 +229,8 @@ class V8_EXPORT_PRIVATE WasmModuleBuilder : public ZoneObject {
   // Writing methods.
   void WriteTo(ZoneBuffer& buffer) const;
 
+  // TODO(titzer): use SignatureMap from signature-map.h here.
+  // This signature map is zone-allocated, but the other is heap allocated.
   struct CompareFunctionSigs {
     bool operator()(FunctionSig* a, FunctionSig* b) const;
   };