DevTools: Support refreshing folders contents in workspace.

Added support for refreshing folder contents by adding new files into workspace.
Refreshing could be initiated by user action on folder context menu.
Also added an experiment for automatical folder contents refresh on window focus.

Review URL: https://chromiumcodereview.appspot.com/22217002

git-svn-id: svn://svn.chromium.org/blink/trunk@155599 bbb929c8-8fbe-4397-9dbb-9b2b20218538