Worker: Specify a unique request id for WorkerScriptFetcher

According to the issue, each browser-initiated request should have a unique
request id.

Bug: 937174
Change-Id: I49f27ef4a65aa98697156a5eb03f20f3f14e867d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623650
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662055}