DevTools: [Sources] Show proper shortcut in the Pause/Resume button tooltip

The action's shortcut descriptors should be used to compute the textual
shortcut representation, rather than an empty array.

R=aandrey, vsevik
BUG=392431

Review URL: https://codereview.chromium.org/385563002

git-svn-id: svn://svn.chromium.org/blink/trunk@177832 bbb929c8-8fbe-4397-9dbb-9b2b20218538