webnn: Enable WebNN implementation on CrOS

This CL uses ML Service on CrOS platform to load and compute the TFLite
model that is converted from WebNN graph. The WebNN Service should be
in browser process because the ML Service connection require as
documented here [1], the connection is initialized in browser process
here [2].

The `ModelLoader` interface needs to be created if needed to load TFLite
model for WebNN graph building, then compute the model with `Model`
interface provided ML Service.

[1] https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/platform2/ml/mojom/machine_learning_service.mojom;l=48?q=ml%2Fmojom%2Fmachine_learning_service.mojom
[2] https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ash/chrome_browser_main_parts_ash.cc;l=1022;drc=a4c7bbe208f9caf621a85cc5e12aa6892d6a5e7a

Bug: 327026293
Change-Id: I893ddde8fe0442da04ed24fe1457695e71341b41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5382266
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: ningxin hu <ningxin.hu@intel.com>
Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Commit-Queue: Junwei Fu <junwei.fu@intel.com>
Cr-Commit-Position: refs/heads/main@{#1278132}