https://bugs.webkit.org/show_bug.cgi?id=122067

Reviewed by Darin Adler.

Add a convenient way for getting the first and last element if it exists.
        
Use it in some places.

* accessibility/AccessibilityNodeObject.cpp:
(WebCore::AccessibilityNodeObject::canvasHasFallbackContent):
* css/CSSFontFaceSource.cpp:
(WebCore::CSSFontFaceSource::getFontData):
* dom/Document.cpp:
(WebCore::Document::childrenChanged):
(WebCore::Document::removeTitle):
* dom/ElementChildIterator.h:
(WebCore::::first):
(WebCore::::last):
* dom/ElementDescendantIterator.h:
(WebCore::::first):
(WebCore::::last):
* html/HTMLFieldSetElement.cpp:
(WebCore::HTMLFieldSetElement::legend):
* html/HTMLLegendElement.cpp:
(WebCore::HTMLLegendElement::associatedControl):
* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::finishParsingChildren):
(WebCore::HTMLMediaElement::selectMediaResource):
* svg/SVGElement.cpp:
(WebCore::SVGElement::title):
* svg/SVGFontFaceElement.cpp:
(WebCore::SVGFontFaceElement::rebuildFontFace):
* svg/graphics/SVGImage.cpp:
(WebCore::SVGImage::hasSingleSecurityOrigin):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156612 268f45cc-cd09-0410-ab3c-d52691b4dbfc