Auto merge of #12330 - stshine:flexitem, r=pcwalton

Implement flexible box layout for row container

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This pull requests implements basic flexible box layout for row container.
It  has implemented most basic flexbox features, including grow, shrink, multi-line, *reverse properties, and alignment under `justify-content`, `align-items`, `align-self`, `align-content`.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X]  There are tests for these changes

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r? @pcwalton

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1 files changed, 7 insertions, 0 deletions

diff --git a/components/layout/model.rs b/components/layout/model.rs
index bb9bb1736fc..f988ef370dd 100644
--- a/components/layout/model.rs
+++ b/components/layout/model.rs
@@ -502,6 +502,13 @@ impl ToGfxMatrix for ComputedMatrix {
     }
 }
 
+// Used to specify the logical direction.
+#[derive(Debug, Clone, Copy)]
+pub enum Direction {
+    Inline,
+    Block
+}
+
 // https://drafts.csswg.org/css2/visudet.html#min-max-widths
 // https://drafts.csswg.org/css2/visudet.html#min-max-heights
 /// A min or max constraint