Default value of |size| IDL attribute of HTMLSelectElement should be 0.

https://html.spec.whatwg.org/multipage/forms.html#dom-select-size
> The size IDL attribute has a default value of zero.

The new behavior is compatible with IE and Firefox.

BUG=415456

Review URL: https://codereview.chromium.org/579133002

git-svn-id: svn://svn.chromium.org/blink/trunk@182223 bbb929c8-8fbe-4397-9dbb-9b2b20218538