[devtools] Allow resolving node objects by backend id.

DOMSnapshot domain users (primarily automation users) don't receive
nodeIds, only backendNodeIds. Still, they'd like to interact with
node objects via JS.

Bug: 546953
Change-Id: Idbce383d9b264038b737bc316620fd308e839510
Reviewed-on: https://chromium-review.googlesource.com/559390
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485242}
6 files changed