Auto merge of #10643 - mbrubeck:empty-span-border, r=pcwalton

Fix handling of borders and padding for empty/stripped inline flows

This forces fragment generation for empty inline flows, if they have borders or padding.  Fixes #10533 and #2001.  Also includes fixes for other bugs that were uncovered by this change; see the individual commit messages for detailed explanations.  r? @pcwalton

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components/style/values.rs

@@ -1662,9 +1662,22 @@ pub mod computed {
     }
 
     impl LengthOrPercentage {
+        #[inline]
         pub fn zero() -> LengthOrPercentage {
             LengthOrPercentage::Length(Au(0))
         }
+
+        /// 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::LengthOrPercentage::*;
+            match *self {
+                Length(Au(0)) | Percentage(0.0) => true,
+                Length(_) | Percentage(_) | Calc(_) => false
+            }
+        }
     }
 
     impl fmt::Debug for LengthOrPercentage {