Add diagnostic code in order to investigate a time format issue.

Add "datetimeformat" attribute to a shadow element for a detected date/time
format. This attribute is visible only in DevTools.

BUG=370498

Review URL: https://codereview.chromium.org/270633002

git-svn-id: svn://svn.chromium.org/blink/trunk@173529 bbb929c8-8fbe-4397-9dbb-9b2b20218538