Web Inspector: make tabbed pane header a relayout boundary.
https://bugs.webkit.org/show_bug.cgi?id=108650

Reviewed by Alexander Pavlov.

Otherwise, its measure width routine causes total reflow.

* inspector/front-end/tabbedPane.css:
(.tabbed-pane-header):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141751 268f45cc-cd09-0410-ab3c-d52691b4dbfc