Remove m_requestEntityBody from XMLHttpRequest

We don't have to hold the FormData instance to be used by createRequest()
as a class member. It's instantiated right before createRequest() call.
We can just allocate it on stack.

BUG=none
R=ricea,tkent

Review URL: https://codereview.chromium.org/314283005

git-svn-id: svn://svn.chromium.org/blink/trunk@175776 bbb929c8-8fbe-4397-9dbb-9b2b20218538