Track rem unit usage in StyleSheetInternal

Track rem unit usage in StyleSheetInternal
https://bugs.webkit.org/show_bug.cgi?id=83923

Reviewed by Andreas Kling.

Currently CSSParser sets usesRemUnits flag directly to Document. It should be set to
the stylesheet instead so parser does not need to know about the document. This also
avoids setting the flag unnecessarily when the stylesheet is not used.

* css/CSSGrammar.y:
* css/CSSStyleSheet.cpp:
(WebCore::StyleSheetInternal::StyleSheetInternal):
* css/CSSStyleSheet.h:
(WebCore::StyleSheetInternal::parserSetUsesRemUnits):
(WebCore::StyleSheetInternal::usesRemUnits):
(StyleSheetInternal):
* dom/Document.cpp:
(WebCore):
(WebCore::checkUsesRemUnits):
(WebCore::Document::updateActiveStylesheets):
* dom/Document.h:
(WebCore::Document::usesRemUnits):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@114168 268f45cc-cd09-0410-ab3c-d52691b4dbfc