Make HTMLInputElement.valueAsDate IDL match current spec.

Marking valueAsDate as nullable to match the current spec 
text. This change does not introduce any JavaScript visible
changes because the current double to Date conversion in
V8Binding.h returns null for all non-finite numbers.
This change sets the stage for allowing v8DateOrNull() to  
be changed to v8DateOrNaN() so that NaN dates can be returned.

BUG=359773
TEST=All existing LayoutTests pass.

Review URL: https://codereview.chromium.org/224523002

git-svn-id: svn://svn.chromium.org/blink/trunk@170803 bbb929c8-8fbe-4397-9dbb-9b2b20218538