Hide HOST_RULE behind SHADOW_DOM flag.

Hide HOST_RULE behind SHADOW_DOM flag.
https://bugs.webkit.org/show_bug.cgi?id=102321

Reviewed by Dimitri Glazkov.

@host @-rules are only available when the rules are declared in styles
in shadow dom trees. So if SHADOW_DOM is disabled, we don't need
@host @-rules.

No new tests. Just hide HOST_RULE behind SHADOW_DOM flag.

* css/StyleResolver.h:
Hide "class ShadowRuleHost".
* css/StyleRule.cpp:
(WebCore::StyleRuleBase::reportMemoryUsage):
(WebCore::StyleRuleBase::destroy):
(WebCore::StyleRuleBase::copy):
(WebCore::StyleRuleBase::createCSSOMWrapper):
* css/StyleRule.h:
(StyleRuleBase):
Hide "Host" type and isHostRule.
(StyleRuleHost):
Hide "class StyleRuleHost".
* css/StyleSheetContents.cpp:
(WebCore::childRulesHaveFailedOrCanceledSubresources):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@137417 268f45cc-cd09-0410-ab3c-d52691b4dbfc