DevTools: [Elements] Do not update sidebar if frame resize event arrives during styles editing

This is regression from r163004.

R=pfeldman, vsevik
BUG=359853

Review URL: https://codereview.chromium.org/227463003

git-svn-id: svn://svn.chromium.org/blink/trunk@171039 bbb929c8-8fbe-4397-9dbb-9b2b20218538