Revert of DevTools: keep widgets in widget hierarchy upon hide, split attach/detach cycle from show/hide.

Revert of DevTools: keep widgets in widget hierarchy upon hide, split attach/detach cycle from show/hide. (patchset #4 id:60001 of https://codereview.chromium.org/2157363006/ )

Reason for revert:
REverting as this broke devtools on Tip-of-tree

[17701:17701:0721/140403:ERROR:CONSOLE(266)] "Uncaught (in promise) NotFoundError: Failed to execute 'insertBefore' on 'Node': The node before which the new node is to be inserted is not a child of this node.", source: chrome-devtools://devtools/bundled/ui/Widget.js (266)


Original issue's description:
> DevTools: keep widgets in widget hierarchy upon hide, split attach/detach cycle from show/hide.
> 
> Committed: https://crrev.com/3239d5a2859023f95bb94774f77ddf903d6280cc
> Cr-Commit-Position: refs/heads/master@{#406767}

TBR=dgozman@chromium.org,pfeldman@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

[lushnikov, 2016-07-21 21:07:14]

Created Revert of DevTools: keep widgets in widget hierarchy upon hide, split
attach/detach cycle from show/hide.