DevTools: Do not show save/save as context menu items for file system files and snippets.

BUG=384363
R=lushnikov

Review URL: https://codereview.chromium.org/335113007

git-svn-id: svn://svn.chromium.org/blink/trunk@176747 bbb929c8-8fbe-4397-9dbb-9b2b20218538