[ios blink] Make webnn work on the device

tflite as a fallback works correctly on iOS in the GPU process.
CoreML model requires file system access to compile a model
and the GPU process is restricted from having file access. We've
asked Apple to provide an update how CoreML can be used in the
sandboxed rendering process.

Bug: 40254930
Change-Id: I72598db882db38fa43d5d04f4c6055cbafb33a87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6543648
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460093}