Show a warning when using sync xhr.

This CL shows a message in developer tools console, warning that the sync xhr may
hamper responsiveness.
This was shown as per the spec suggestion: https://xhr.spec.whatwg.org/#sync-warning

BUG=392311

Review URL: https://codereview.chromium.org/585873002

git-svn-id: svn://svn.chromium.org/blink/trunk@184788 bbb929c8-8fbe-4397-9dbb-9b2b20218538