Emscripten's headers live at <webgpu/*>; this makes it so projects don't
have to ifdef which #include path to use.
Bug: dawn:160
Change-Id: I39efee44447470551a78db0abd062f4a2fe9083a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/26020
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>