DevTools: [Elements] Fix click handling for the sidebar tabbed pane chevron

R=aandrey, eustas, pfeldman@chromium.org
BUG=330497

Review URL: https://codereview.chromium.org/112903004

git-svn-id: svn://svn.chromium.org/blink/trunk@164288 bbb929c8-8fbe-4397-9dbb-9b2b20218538