DevTools: add-to-watch shortcut

This patch adds shortcut Ctrl-Shift-A to the sources panel which
adds current selection to watch.

BUG=234954

Review URL: https://codereview.chromium.org/25521003

git-svn-id: svn://svn.chromium.org/blink/trunk@158806 bbb929c8-8fbe-4397-9dbb-9b2b20218538