[wasm] [cleanup] Avoid creation of temporary String objects

We were converting an int to a Smi, calling ToString to create a String
object, then appending this String to an IncrementalStringBuilder.
It's much easier and more efficient to just sprintf to a local buffer
and append that instead.

R=titzer@chromium.org, ahaas@chromium.org
BUG=v8:5822

Change-Id: I9302a07971cfd32350d69b1b8f182d0ba7245b77
Reviewed-on: https://chromium-review.googlesource.com/454018Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43857}

src/messages.cc

@@ -686,13 +686,14 @@ MaybeHandle<String> WasmStackFrame::ToString() {
 
   builder.AppendCString(" (<WASM>[");
 
-  Handle<Smi> ix(Smi::FromInt(wasm_func_index_), isolate_);
-  builder.AppendString(isolate_->factory()->NumberToString(ix));
+  char buffer[16];
+  SNPrintF(ArrayVector(buffer), "%u", wasm_func_index_);
+  builder.AppendCString(buffer);
 
   builder.AppendCString("]+");
 
-  Handle<Object> pos(Smi::FromInt(GetPosition()), isolate_);
-  builder.AppendString(isolate_->factory()->NumberToString(pos));
+  SNPrintF(ArrayVector(buffer), "%d", GetPosition());
+  builder.AppendCString(buffer);
   builder.AppendCString(")");
 
   return builder.Finish();