Get rid of custom constructor for FormData

Get rid of custom constructor for FormData and let the generator produces its
code instead. There does not appear to be any difference between the custom
code and the generated one.

R=haraken
TEST=http/tests/local/formdata

Review URL: https://codereview.chromium.org/63133002

git-svn-id: svn://svn.chromium.org/blink/trunk@161498 bbb929c8-8fbe-4397-9dbb-9b2b20218538