CSS Parser: Mark on stylesheets whether the first rule was valid

      This makes the new CSS parser call setHasSyntacticallyValidCSSHeader so
that we can determine whether we should allow stylesheets to be used.
A StyleRuleCharset class is added so that we can properly signal whether
a rule was valid (now consumeAtRule and consumeQualifiedRule will return
nullptr exactly when the rule is valid). For more context, the logic
regarding disallowing certain stylesheets is discussed in bug 9877.

BUG=330389

Review URL: https://codereview.chromium.org/1018213004

git-svn-id: svn://svn.chromium.org/blink/trunk@192339 bbb929c8-8fbe-4397-9dbb-9b2b20218538