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}