Devirtualize Document class type checking

Devirtualize Document class type checking
https://bugs.webkit.org/show_bug.cgi?id=115755

Reviewed by Benjamin Poulain.

Merge https://chromium.googlesource.com/chromium/blink/+/dae5adc768d5ce6bff301df6515745da8ea24950

Document has a bunch of virtual bool is*Document() methods on it, but it also has
two bools for XHTML and HTML documents which is silly. We can merge them all
together into an enum of values and devirtualize the type checking methods.

* dom/Document.cpp:
(WebCore::Document::Document):
(WebCore::Document::createElement):
* dom/Document.h:
(WebCore::Document::create):
(WebCore::Document::createXHTML):
(WebCore::Document::isHTMLDocument):
(WebCore::Document::isXHTMLDocument):
(WebCore::Document::isImageDocument):
(WebCore::Document::isSVGDocument):
(WebCore::Document::isPluginDocument):
(WebCore::Document::isMediaDocument):
(WebCore::Document):
* html/HTMLDocument.cpp:
(WebCore::HTMLDocument::HTMLDocument):
* html/HTMLDocument.h:
(WebCore::HTMLDocument):
* html/ImageDocument.cpp:
(WebCore::ImageDocument::ImageDocument):
* html/ImageDocument.h:
(WebCore::ImageDocument):
* html/MediaDocument.cpp:
(WebCore::MediaDocument::MediaDocument):
* html/MediaDocument.h:
(WebCore::MediaDocument):
* html/PluginDocument.cpp:
(WebCore::PluginDocument::PluginDocument):
* html/PluginDocument.h:
(WebCore::PluginDocument):
* loader/PlaceholderDocument.h:
(WebCore::PlaceholderDocument::PlaceholderDocument):
* svg/SVGDocument.cpp:
(WebCore::SVGDocument::SVGDocument):
* svg/SVGDocument.h:
(WebCore::SVGDocument):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@149705 268f45cc-cd09-0410-ab3c-d52691b4dbfc