Revert of DevTools: fight responsive design-related paint storms. (https://codereview.chromium.org/346213002/)

Reason for revert:
This breaks drawer resizing.

Original issue's description:
> DevTools: fight responsive design-related paint storms.
> This also adds missing flex to the tabbed-pane that I think is Ok to fix in a generic manner. Wdyt?
> R=dgozman,vsevik
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176925

TBR=dgozman@chromium.org,vsevik@chromium.org,pfeldman@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/355033002

git-svn-id: svn://svn.chromium.org/blink/trunk@176979 bbb929c8-8fbe-4397-9dbb-9b2b20218538