Remove the scoped tracker for FileStream::Context::Read on Windows.
Remove the scoped tracker for FileStream::Context::Read on Windows.

The underlying bug with ReadFile causing the IO thread to block at times has been addressed by moving
the call to the worker pool.

BUG=423948

Review URL: https://codereview.chromium.org/913993002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#315720}

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