DevTools: Add user metrics for saving a file to workspace

Chromium counterpart: https://codereview.chromium.org/423713008/

R=pfeldman
BUG=399449
NOTRY=TRUE

Review URL: https://codereview.chromium.org/439463002

git-svn-id: svn://svn.chromium.org/blink/trunk@179444 bbb929c8-8fbe-4397-9dbb-9b2b20218538