Reland "Roll emsdk to 4.0.20"

This reverts commit c25f72b35e5a3cca81d201c58091738eaf902694.

Reason for revert:
This is now fixed with David's fix applied.
(It also incidentally was fixed by include path changes in
https://dawn-review.googlesource.com/c/dawn/+/276214 - the build config
is quite tangled, but unfortunately it's not easy to detangle.)

Original change's description:
> Revert "Roll emsdk to 4.0.20"
>
> This reverts commit cf78ce87c4b1b066e651df5560b7cdf3f502bfc0.
>
> Reason for revert: breaks on GN build (I forgot this was why it
> needed to be reverted the first time)
>
> Original change's description:
> > Roll emsdk to 4.0.20
> >
> > gclient setdep -r third_party/emsdk@e4fe26ef59168ff44f4c23c466e497bf60b3411e
> >
> > Bug: 456628846
> > Change-Id: I7b7f38d409abb9f33f8bf1deef5af4523f132f58
> > Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/276940
> > Commit-Queue: Kai Ninomiya <kainino@chromium.org>
> > Reviewed-by: Fr <beaufort.francois@gmail.com>
>
> # Not skipping CQ checks because original CL landed > 1 day ago.
>
> Bug: 456628846
> Change-Id: Ia909e151a90fa434d566a6be64f5add73e0c8d18
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/277415
> Reviewed-by: Fr <beaufort.francois@gmail.com>
> Commit-Queue: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
> Auto-Submit: Kai Ninomiya <kainino@chromium.org>
> Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 456628846
Change-Id: I1f4a25d011dc29d8982bc5b6c13bc23eb40036f9
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/278636
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>