DevTools: Tabbed pane now measures tab element sizes after zoom change.

BUG=363980
R=pfeldman

Review URL: https://codereview.chromium.org/254393005

git-svn-id: svn://svn.chromium.org/blink/trunk@172486 bbb929c8-8fbe-4397-9dbb-9b2b20218538