style: Centralize a bit invalid value error reporting.

Also, buffer the errors, since we're going to want to look at the whole
declaration block to skip reporting them.

This shouldn't change behavior, just moves some work to the caller, and defers a
bit the work so that it happens only when error reporting is enabled.

Differential Revision: https://phabricator.services.mozilla.com/D30200

components/style/parser.rs

@@ -136,6 +136,12 @@ impl<'a> ParserContext<'a> {
             .expect("Rule type expected, but none was found.")
     }
 
+    /// Returns whether CSS error reporting is enabled.
+    #[inline]
+    pub fn error_reporting_enabled(&self) -> bool {
+        self.error_reporter.is_some()
+    }
+
     /// Record a CSS parse error with this context’s error reporting.
     pub fn log_css_error(&self, location: SourceLocation, error: ContextualParseError) {
         let error_reporter = match self.error_reporter {