Revert of DevTools: Fix debug_devtools=1 mode "main" extensions loading after r182328 (patchset #1 id:1 of https://codereview.chromium.org/593673002/)

Reason for revert:
Broke the following tests:
inspector/initial-modules-load.html
inspector/open-with-rendering-option-enabled.html

Original issue's description:
> DevTools: Fix debug_devtools=1 mode "main" extensions loading after r182328
> 
> R=dgozman,lushnikov
> NOTRY=true
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=182405

TBR=dgozman@chromium.org,lushnikov@chromium.org,apavlov@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/593703002

git-svn-id: svn://svn.chromium.org/blink/trunk@182409 bbb929c8-8fbe-4397-9dbb-9b2b20218538