Auto merge of #16541 - sbwtw:fix_calc_behavior, r=stshine

Add calc method for CalcLengthOrPercentage

* Add calc method to calculate formula value with parent size
* Add unit test case

<!-- Please describe your changes on the following line: -->
Please see this issue: https://github.com/servo/servo/issues/15986

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15986 (github issue number if applicable).

<!-- Either: -->
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/16541)
<!-- Reviewable:end -->

components/style/values/computed/length.rs

@@ -76,6 +76,16 @@ impl CalcLengthOrPercentage {
     pub fn percentage(&self) -> CSSFloat {
         self.percentage.unwrap_or(0.)
     }
+
+    /// If there are special rules for computing percentages in a value (e.g. the height property),
+    /// they apply whenever a calc() expression contains percentages.
+    pub fn to_computed(&self, container_len: Option<Au>) -> Option<Au> {
+        match (container_len, self.percentage) {
+            (Some(len), Some(percent)) => Some(self.length + len.scale_by(percent)),
+            (_, None) => Some(self.length),
+            _ => None,
+        }
+    }
 }
 
 impl From<LengthOrPercentage> for CalcLengthOrPercentage {