* Add WebPreference::interactive_form_validation_enabled

  The default value is true.

* Add --disable-interactive-form-validation flag and about:flags interface

The interactive form validation feature might cause web site
compatibility issues.  So we provide a way to disable it just in case.

BUG=45804,54368
TEST=manual; Open the URL "data:text/html,<form><input required></form>", click
  the input field, and press ENTER key.
 - A validation message is shown by default
 - If you run Chrome with --disable-interactive-form-validation, no validation
   message is shown and the form is submitted.
 - If you enable "Disable HTML5 interacrive form validation" flag on
   about:flags, no validation message is shown and the form is submitted.
Review URL: http://codereview.chromium.org/6329005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@71902 0039d316-1c4b-4281-b951-d872f2087c98