DevTools: [Elements] Make the sidebar panes tabbed pane header a resizer
DevTools: [Elements] Make the sidebar panes tabbed pane header a resizer

Now that the chevron issue is universally fixed in r164293,
we can roll in the reverted part of r163547

R=aandrey, eustas, vsevik

Review URL: https://codereview.chromium.org/115723004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@164296 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs