[dawn][emscripten] Fixes some signed to unsigned issues in JS functions.

- Introduces two helpers to both document and make it clear when unsigned
  WASM integers should be interpreted as unsigned. For a lot of the cases,
  the values are expected to be smaller than 2^31 and hence we can add an
  assert that they are <= 0 before passing the value into JS functions.
  In some cases, the values passed from WASM may be larger than 2^31 and
  in those cases, we need to apply the >>>= 0 workaround before passing
  the values into JS functions.

Change-Id: I2e3bddbb848f3e4950724173e62b8c7f14e93c95
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/255894
Auto-Submit: Loko Kung <lokokung@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>