webnn: Use base::test::InsecureRandomGenerator to generate fuzzer inputs

std::uniform_int_distribution is too slow but we can just use
base::InsecureRandomGenerator directly and generate 64-bit chunks of
data at once.

While we're here, the RNG is now seeded only once for all the test cases
(since there's only one seed parameter anyways) rather than once per
input tensor.

Fixed: 421228413
Change-Id: I84637613824be2a34d0c116d4f54c35c730ac7ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6613302
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1468419}