ScopedStyleResolver now holds on CSSStyleSheets instead 
of StyleSheetContents. Plumbing added so that every MatchedRule 
instance has a reference to the parent CSSStyleSheet where 
this rule comes from.

ElementRuleCollector::appendCSSOMWrappers method now either
finds a wrapping CSSRule in the author CSSStyleSheet or creates
a new CSSRule without parent style sheet in case of UserAgent rule.

InspectorCSSOMWrappers are not needed any more and removed.

InspectorCSSAgent treats any rule without a parentStyleSheet as a rule coming
from User Agent.

Review URL: https://codereview.chromium.org/115153005

git-svn-id: svn://svn.chromium.org/blink/trunk@164650 bbb929c8-8fbe-4397-9dbb-9b2b20218538