Polish code for r189579.

Polish code for r189579.
https://bugs.webkit.org/show_bug.cgi?id=149131

Reviewed by Chris Dumez.

Use more self-explaining function name "characterSetWithUTF8Fallback" instead of
"characterSetForBindings". Optimize the way to get text encoding.

* dom/Document.cpp:
(WebCore::Document::characterSetWithUTF8Fallback):
(WebCore::Document::encoding): Deleted.
(WebCore::Document::characterSetForBindings): Deleted.
* dom/Document.h:
(WebCore::Document::encoding):
(WebCore::Document::textEncoding):
* dom/Document.idl:
* loader/DocumentWriter.cpp:
(WebCore::DocumentWriter::createDecoderIfNeeded):
* loader/FormSubmission.cpp:
(WebCore::encodingFromAcceptCharset):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@189771 268f45cc-cd09-0410-ab3c-d52691b4dbfc