Bug 1374233 - Part 1: Add NonNegativeNumber and GreaterThanOrEqualToOneNumber.

NonNegativeNumber: for -moz-box-flex, flex-grow, and flex-shrink.
GreaterThanOrEqualToOneNumber: for stroke-miterlimit.

MozReview-Commit-ID: Kgbt99BPdVA

components/layout/flex.rs

@@ -137,8 +137,8 @@ impl FlexItem {
             min_size: Au(0),
             max_size: MAX_AU,
             index: index,
-            flex_grow: flex_grow,
-            flex_shrink: flex_shrink,
+            flex_grow: flex_grow.into(),
+            flex_shrink: flex_shrink.into(),
             order: order,
             is_frozen: false,
             is_strut: false