webnn: Allow an MLGraphBuilder to build at most one MLGraph #47285

Safari results

Results have successfully been scraped and added to wpt.fyi.

Uh-oh - it looks like there are some newly-failing results when we compared the affected tests
to the latest run against the master branch.

Run	Spec
`master`	safari-199 preview-mac-14.5@084f993d23
`084f993`	safari-199 preview-mac-14.5@084f993d23

Test	`master`	`084f993`
/webnn/validation_tests/clamp.https.any.html	1 / 8	0 / 8
/webnn/validation_tests/clamp.https.any.worker.html	1 / 8	0 / 8
/webnn/validation_tests/concat.https.any.html	1 / 12	0 / 12
/webnn/validation_tests/concat.https.any.worker.html	1 / 12	0 / 12
/webnn/validation_tests/constant-changed-buffer.https.any.html	1 / 4	0 / 4
/webnn/validation_tests/constant-changed-buffer.https.any.worker.html	1 / 4	0 / 4
/webnn/validation_tests/constant.https.any.html	9 / 19	0 / 19
/webnn/validation_tests/constant.https.any.worker.html	9 / 19	0 / 19
/webnn/validation_tests/elementwise-unary.https.any.html	1 / 40	0 / 40
/webnn/validation_tests/elementwise-unary.https.any.worker.html	1 / 40	0 / 40
/webnn/validation_tests/elu.https.any.html	2 / 11	0 / 11

And 9 others...