[DevTools] Support multiple sessions in Target domain

BUG=590878

Change-Id: Idcebf0336623322813b4cdfac66183d37c28f2bd
Reviewed-on: https://chromium-review.googlesource.com/573411
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488395}
13 files changed