DevTools: [Styles] Avoid selector editor commit upon focused element style changes

Selector editor never set the _isEditingStyle flag on the StylesSidebarPane instance.

R=aandrey, eustas, vsevik
BUG=326462

Review URL: https://codereview.chromium.org/108243002

git-svn-id: svn://svn.chromium.org/blink/trunk@163433 bbb929c8-8fbe-4397-9dbb-9b2b20218538