[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 3765cad565e62194ba92fb3f9fe9cf341ad05329..4a871b2ef505aa0ff2216962c9475d876fd73784 100644
--- a/src/wasm/wasm-module-builder.h
+++ b/src/wasm/wasm-module-builder.h
@@ -222,6 +222,8 @@ 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;
   };