DevTools: "Jump between editing locations" experiment

This patch introduces "Jump between editing locations" experiment. It
adds "Alt+" and "Alt-" shortcuts to the scripts panel to navigate between
editing locations. An editing location is considered to be switched if
one of the following happens:
- A user clicks with his mouse and changes cursor position
- Position in editor was highlighted and cursor was moved there (e.g.
due to "Goto function" outline dialog)
- A new file was opened

BUG=281507
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/23474010

git-svn-id: svn://svn.chromium.org/blink/trunk@165395 bbb929c8-8fbe-4397-9dbb-9b2b20218538