[wasm] WasmGrowMemory runtime function must return Smi

The WasmGrowMemory builtin (defined in builtins-wasm-gen.cc) expects
the runtime function to return a Smi. Thus ensure that we are really
always returning a Smi.

R=mstarzinger@chromium.org

Change-Id: I153d75a2ab8773adbcce01e9aa125a234c93d077
Reviewed-on: https://chromium-review.googlesource.com/1131737Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54411}

src/runtime/runtime-wasm.cc

@@ -80,8 +80,11 @@ RUNTIME_FUNCTION(Runtime_WasmGrowMemory) {
   DCHECK_NULL(isolate->context());
   isolate->set_context(instance->native_context());
 
-  return *isolate->factory()->NewNumberFromInt(WasmMemoryObject::Grow(
-      isolate, handle(instance->memory_object(), isolate), delta_pages));
+  int ret = WasmMemoryObject::Grow(
+      isolate, handle(instance->memory_object(), isolate), delta_pages);
+  // The WasmGrowMemory builtin which calls this runtime function expects us to
+  // always return a Smi.
+  return Smi::FromInt(ret);
 }
 
 RUNTIME_FUNCTION(Runtime_ThrowWasmError) {