DevTools: [Elements] Implement "Copy CSS Path" context menu item for elements

R=pfeldman, vsevik, aandrey, eustas
BUG=277286

Review URL: https://codereview.chromium.org/75253002

git-svn-id: svn://svn.chromium.org/blink/trunk@162552 bbb929c8-8fbe-4397-9dbb-9b2b20218538