The code to look for an ancestor form element is duplicated in three different places

The code to look for an ancestor form element is duplicated in three different places
https://bugs.webkit.org/show_bug.cgi?id=120391

Reviewed by Darin Adler.

Unduplicated the code by putting a single implementation in HTMLFormElement.cpp.

* WebCore.order:
* html/FormAssociatedElement.cpp:
(WebCore::FormAssociatedElement::findAssociatedForm):
(WebCore::FormAssociatedElement::formAttributeChanged):
* html/HTMLElement.cpp:
(WebCore::HTMLElement::virtualForm):
* html/HTMLElement.h:
* html/HTMLFormControlElement.cpp:
(WebCore::HTMLFormControlElement::HTMLFormControlElement):
* html/HTMLFormElement.cpp:
(WebCore::HTMLFormElement::findClosestFormAncestor):
* html/HTMLFormElement.h:
* html/HTMLImageElement.cpp:
(WebCore::HTMLImageElement::insertedInto):
* html/HTMLObjectElement.cpp:
(WebCore::HTMLObjectElement::HTMLObjectElement):
* html/parser/HTMLTreeBuilder.cpp:
(WebCore::HTMLTreeBuilder::HTMLTreeBuilder):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@154801 268f45cc-cd09-0410-ab3c-d52691b4dbfc