Revert of DevTools: move flow events tracking to TracingModel, support cross-threads case (patchset #2 id:20001 of https://codereview.chromium.org/2746153005/ )

Reason for revert:
Breaks flow events completely.

Original issue's description:
> DevTools: move flow events tracking to TracingModel, support cross-threads case
>
> This implements support for tracking flow events in TracingModel and correctly supports the spec in terms of id matching for flow events.
>
> Review-Url: https://codereview.chromium.org/2746153005
> Cr-Commit-Position: refs/heads/master@{#457245}
> Committed: https://chromium.googlesource.com/chromium/src/+/56024c61ffdabb977c7645098cb4654c6a1fdcc1

TBR=alph@chromium.org,caseq@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.chromium.org/2750133003
Cr-Commit-Position: refs/heads/master@{#457295}
5 files changed