DevTools: Inspector should be informed when viewport rule creation failed.

The test added here is correct but passes even without this change because tests are run with experimental web platform features enabled by default.

BUG=278289

Review URL: https://chromiumcodereview.appspot.com/23514032

git-svn-id: svn://svn.chromium.org/blink/trunk@157128 bbb929c8-8fbe-4397-9dbb-9b2b20218538