[DevTools] Support multiple sessions in remote debugging

Bug: 590878
Change-Id: I5606ba2082e7ea81b95547b8b28f476ce3f59b43
Reviewed-on: https://chromium-review.googlesource.com/630564


Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501361}