AX: Implement CSS -webkit-alt property (text alternative for generated content pseudo-elements ::before and ::after)
https://bugs.webkit.org/show_bug.cgi?id=120188

Reviewed by Dean Jackson.

Source/WebCore: 

Add a -webkit-alt CSS property that can be used to label Image content or Text content for accessibility clients.

To accomplish this, it sets the string in the RenderStyle. Then when the ContentData creates an anonymous renderer,
it sets that string on the TextFragment or RenderImage, which can be queried by accessibility code.

Test: platform/mac/accessibility/webkit-alt-for-css-content.html

* accessibility/AccessibilityNodeObject.cpp:
(WebCore::AccessibilityNodeObject::alternativeText):
* accessibility/AccessibilityRenderObject.cpp:
(WebCore::AccessibilityRenderObject::textUnderElement):
(WebCore::objectInclusionFromAltText):
(WebCore::AccessibilityRenderObject::computeAccessibilityIsIgnored):
* css/CSSComputedStyleDeclaration.cpp:
(WebCore::altTextToCSSValue):
(WebCore::ComputedStyleExtractor::propertyValue):
* css/CSSParser.cpp:
(WebCore::CSSParser::parseValue):
(WebCore::CSSParser::parseAlt):
* css/CSSParser.h:
* css/CSSPropertyNames.in:
* css/StyleResolver.cpp:
(WebCore::StyleResolver::applyProperty):
* rendering/RenderImage.h:
(WebCore::RenderImage::altText):
(WebCore::RenderImage::setAltText):
* rendering/RenderTextFragment.h:
* rendering/style/ContentData.cpp:
(WebCore::ImageContentData::createRenderer):
(WebCore::TextContentData::createRenderer):
* rendering/style/ContentData.h:
(WebCore::ContentData::setAltText):
(WebCore::ContentData::altText):
* rendering/style/RenderStyle.cpp:
(WebCore::RenderStyle::setContent):
(WebCore::RenderStyle::setContentAltText):
(WebCore::RenderStyle::contentAltText):
* rendering/style/RenderStyle.h:
* rendering/style/StyleRareNonInheritedData.h:

LayoutTests: 

* platform/mac/accessibility/webkit-alt-for-css-content-expected.txt: Added.
* platform/mac/accessibility/webkit-alt-for-css-content.html: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@159591 268f45cc-cd09-0410-ab3c-d52691b4dbfc