message: wasm: add local _emscripten_get_heap_size.
  This is the same as local getTotalMemory to help with per-thread VMs,
  though both are short-term workarounds, really.
diff: @@ -311,6 +311,8 @@ uint32_t getTotalMemoryHandler(void *) {
  return 0x7FFFFFFF;
  }
  +uint32_t _emscripten_get_heap_sizeHandler(void*) { return getTotalMemoryHandler(nullptr); }
  +
  } // namespace
  // Shared Data
  @@ -717,6 +719,7 @@ Wasm::Wasm(absl::string_view vm, ThreadLocal::SlotAllocator&) {
  _REGISTER(httpCall);
  _REGISTER(getTotalMemory);
  + _REGISTER(_emscripten_get_heap_size);
  #undef _REGISTER
  }
  }
  