Expose platform thread id on WebThread

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.

BUG=401895

Review URL: https://codereview.chromium.org/514193002

git-svn-id: svn://svn.chromium.org/blink/trunk@181562 bbb929c8-8fbe-4397-9dbb-9b2b20218538