[DevTools] Rework flatten protocol handling

- Merge TargetRegistry into DevToolsSession.
- Allow flatten children on any targets.

Bug: 775132
Change-Id: Idaafb019f9881a937f3e4b2d1abab7a3e89af06e
Reviewed-on: https://chromium-review.googlesource.com/c/1316748
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605580}
19 files changed