FormDataBuilder should not use Document
https://bugs.webkit.org/show_bug.cgi?id=138172

Reviewed by Alexey Proskuryakov.

It's only used by encodingFromAcceptCharset() to fallback to
document input encoding. That method is only used by
FormSubmission::create(), so it could be moved as a static
function to FormSubmission.cpp.

* loader/FormSubmission.cpp:
(WebCore::encodingFromAcceptCharset):
(WebCore::FormSubmission::create):
* platform/network/FormDataBuilder.cpp:
(WebCore::FormDataBuilder::encodingFromAcceptCharset): Deleted.
* platform/network/FormDataBuilder.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@175369 268f45cc-cd09-0410-ab3c-d52691b4dbfc