webnn: Fix the issue that the ResNet model fails to run

The tflite PAD operator needs signed data type of paddings, so that the
explicit padding can work with this CL.

Bug: 331856216
Change-Id: Ifa544d66d81cecece7158574b77d9a14576fad95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402861


Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: ningxin hu <ningxin.hu@intel.com>
Commit-Queue: Junwei Fu <junwei.fu@intel.com>
Cr-Commit-Position: refs/heads/main@{#1280576}