WebNN: Support WebNN Constant operand in mojo.

There is a `BigBuffer` in mojo for each constant weight data, all
array buffers will be uploaded into one D3D 12 resource for DirectML
backend, the constant data is specified with
DML_TENSOR_FLAG_OWNED_BY_DML flag when creating the tensor desc.

The number of input buffer binding for graph initialization must
exactly match the number of inputs sub constants, and the order of the
DML_BUFFER_BINDING array is inputs, and then constants.

Bug: 1273291
Change-Id: I98b2b6516acbb1eef98883807a6b2f2e9b612653
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4805555
Reviewed-by: ningxin hu <ningxin.hu@intel.com>
Commit-Queue: Junwei Fu <junwei.fu@intel.com>
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1191129}