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Before this CL, LocalFileSystem was supplied to WorkerClients on the
main thread, and then it was passed to a worker thread.

After this CL, LocalFileSystem is supplied to WorkerGlobalScope on the worker
thread. This reduces the usage of WorkerClients, and slightly simplifies
worker startup sequence.

Bug: 1009854
Change-Id: If02556fd161fd90f3f84d3d02f253f1e842dc3e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833014Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702290}