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

Review URL: https://codereview.chromium.org/346213002

git-svn-id: svn://svn.chromium.org/blink/trunk@176925 bbb929c8-8fbe-4397-9dbb-9b2b20218538