DevTools: [Elements] fix ctrl-z behavior.

There used to be an assumption that ctrl-z event is coming from elements
tree outline. This turned out to be wrong in case of focused styles
sidebar pane, which caused ctrl-z event to be ignored.

BUG=405058
R=pfeldman

Review URL: https://codereview.chromium.org/484223002

git-svn-id: svn://svn.chromium.org/blink/trunk@180582 bbb929c8-8fbe-4397-9dbb-9b2b20218538