webnn: Make MLOperandDescriptor.shape a required dictionary member
See webmachinelearning/webnn#758
Also removes the temporary support for passing "dimensions" which was
added in https://crrev.com/e7e99aa5
Bug: 365813262
Cq-Include-Trybots: luci.chromium.try:mac14-blink-rel,win11-blink-rel
Change-Id: Ib714ae540da7fbd7d55365dc739bfb8dbf266406
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5850659
Reviewed-by: ningxin hu ningxin.hu@intel.com
Commit-Queue: ningxin hu ningxin.hu@intel.com
Auto-Submit: Austin Sullivan asully@chromium.org
Cr-Commit-Position: refs/heads/main@{#1369192}