webnn: Validate that input and output names are unique

Validates that the Mojo GraphInfo structure contains only unique input
and output operand names. It is impossible to construct a graph with
duplicate names using the JavaScript interface but a malicious renderer
could pass whatever values it likes. Conversion to platform-specific
model formats assumes uniqueness.

Fixed: 332751951
Change-Id: I3c6edfcb51c04b1c5b87e4bbfc85e6c49fdf6232
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5420667


Auto-Submit: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282321}