webnn: Allow an MLGraphBuilder to build at most one MLGraph
See webmachinelearning/webnn#717
Bug: 354724062
Change-Id: I8ac2bf94b1f5a0db93a042babdc2556eab35034a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5684454
Reviewed-by: Reilly Grant reillyg@chromium.org
Commit-Queue: Austin Sullivan asully@chromium.org
Cr-Commit-Position: refs/heads/main@{#1332702}