FormData allow empty names to append

This CL allows FormData submitted with empty name field to be appended. According to xhr spec https://xhr.spec.whatwg.org/#interface-formdata name field should be appended as it is i.e without checking whether it is empty. 
Firefox and IE behaves according to spec and allow empty names field to be appended. This CL makes Blink to be compatible with spec and other browsers.

BUG=415804

Review URL: https://codereview.chromium.org/609733004

git-svn-id: svn://svn.chromium.org/blink/trunk@183660 bbb929c8-8fbe-4397-9dbb-9b2b20218538