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To support this, we issue inspect element command to the agent
beforehand, and dispatch it in the next session which attaches
(or the existing one if any). This moves InspectElement from
DevToolsSession interface to DevToolsAgent interface, as there
might be no session yet.

Note this could fail in case of multiple sessions, one of which is
DevTools frontend, by choosing the wrong one. This is a rare
case though, so we can tolerate that.

TBR=alexclarke@chromium.org

Bug: 755515, 804100
Change-Id: I331bfb7f5410868fd47bfe8d558b54b7a30de00c
Reviewed-on: https://chromium-review.googlesource.com/881522Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532220}