Reland "webnn: Fall back to WebNN service if built without XNNPACK"

This is a reland of commit 4e4028aa878e864c4b887a32ec343057c32c58c7

The original CL forgot to flag-guard one MLGraphMojo callsite,
resulting in test expectations failing with a different error code
than before

Original change's description:
> webnn: Fall back to WebNN service if built without XNNPACK
>
> Currently, the developer must specify a "gpu" or "npu" device type to
> target the WebNN Service. This CL allows for targeting the WebNN Service
> regardless of the device type (i.e. "cpu", too)
>
> Note that the WebNN Service must still be explicitly enabled to be used
>
> Bug: 325612086, 41486052
> Change-Id: I3b7471b4f2f7f27a7f732406f8249deac42f5a51
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5439712
> Reviewed-by: Reilly Grant <reillyg@chromium.org>
> Commit-Queue: Austin Sullivan <asully@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1284808}

Cq-Include-Trybots: luci.chromium.try:win11-arm64-blink-rel
Bug: 325612086, 41486052
Change-Id: Iec059c56cad2a4edca67f279b6fddd98658a9abc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5443343
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Auto-Submit: Austin Sullivan <asully@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1285185}