webnn: expose graph.devices

Expose graph.devices from idl, this is populated by webnn backend on
create graph success. Follow up CLs will get the devices information
for each backend.

Change-Id: I258071db2cba83f38a1e6b24eab68d800063026b
Bug: 418031018
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6548417
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Phillis Tang <phillis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1463679}