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DevTools code needs access to platform thread id in order to collect Worker thread ids and based on them differentiate worker threads in recorded trace events.

Blink part of the change: https://codereview.chromium.org/514193002/

BUG=401895

Review URL: https://codereview.chromium.org/515243002

Cr-Commit-Position: refs/heads/master@{#294345}