layout: Check flow descendants of inline block fragments to find their

baselines when aligning inline fragments per CSS 2.1 § 10.8.1.

components/style/values.rs

@@ -1837,6 +1837,21 @@ pub mod computed {
         Auto,
         Calc(CalcLengthOrPercentage),
     }
+
+    impl LengthOrPercentageOrAuto {
+        /// Returns true if the computed value is absolute 0 or 0%.
+        ///
+        /// (Returns false for calc() values, even if ones that may resolve to zero.)
+        #[inline]
+        pub fn is_definitely_zero(&self) -> bool {
+            use self::LengthOrPercentageOrAuto::*;
+            match *self {
+                Length(Au(0)) | Percentage(0.0) => true,
+                Length(_) | Percentage(_) | Calc(_) | Auto => false
+            }
+        }
+    }
+
     impl fmt::Debug for LengthOrPercentageOrAuto {
         fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
             match *self {