style: Use Rust sizes for flex-basis, width, height, and their min/max properties.

Really sorry for the size of the patch :(

Only intentional behavior change is in the uses of HasLengthAndPercentage(),
where it's easier to do the right thing. The checks that used to check for
(IsCalcUnit() && CalcHasPercentage()) are wrong since bug 957915.

Differential Revision: https://phabricator.services.mozilla.com/D19553

components/style/values/generics/flex.rs

@@ -19,9 +19,12 @@
     ToComputedValue,
     ToCss,
 )]
-pub enum FlexBasis<Width> {
+#[repr(C)]
+pub enum GenericFlexBasis<S> {
     /// `content`
     Content,
     /// `<width>`
-    Width(Width),
+    Size(S),
 }
+
+pub use self::GenericFlexBasis as FlexBasis;