Do not show date format warnings for Modernizr.

This is a follow-up of Blink r180341 [1].

Modernizr sets an invalid string to a date input element to check if date input
type is actually supported. We'd like to avoid to show warning messages for such
feature detection. So,

- We don't show the warning if value IDL attribute is updated and the element is
  not visible.
- We don't show the warning if type attribute is changed with a dirty invalid
  value.

Also, use WarningMessageLevel instead of ErrorMessageLevel.

[1] http://src.chromium.org/viewvc/blink?view=revision&revision=180341

BUG=397952

Review URL: https://codereview.chromium.org/491703002

git-svn-id: svn://svn.chromium.org/blink/trunk@180840 bbb929c8-8fbe-4397-9dbb-9b2b20218538