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(1) StyleEngine has two hashmaps, one is a mapping from css text to StyleSheetContents and the other is a mapping from StyleSheetContents to css text. When destroying StyleSheetContents, using the above mapping to remove css text.
The hashmaps are only for non-quirks document. Because non-quirks document cannot share styles with quirks document.
 
(2) made StyleSheetContents::checkLoaded to invoke new clients' sheetLoaded. To do so,
  (2-1) moved m_loadCompleted flag from StyleSheetContents to CSSStyleSheet,
  (2-2) added more "protect" to StyleSheetContents::checkLoaded to avoid CSSStyleSheet (i.e. client) and ownerNode being deleted by scripts, and
  (2-3) modified StyleSheetContents::loadCompleted() to see whether all clients completed loading.

(3) LinkStyle::setCSSStyleSheet should use StyleSheetContents' checkLoaded. Because now checkLoaded invokes only new clients' sheetLoaded. We don't need to directly invoke LinkStyle::sheetLoaded (this breaks m_loadCompleted flag and causes assertion failure). 

BUG=308781
TEST=all existing tests covers, so the tests should pass.

Review URL: https://codereview.chromium.org/28553005

git-svn-id: svn://svn.chromium.org/blink/trunk@165139 bbb929c8-8fbe-4397-9dbb-9b2b20218538