Auto merge of #13148 - Manishearth:calc-null, r=emilio

Don't use mem::uninitialized() for making calc values

We had code relying on `nsStyleCoord::set()` being leaky (like it was
before we patched up the calc bindings). Added `nsStyleCoord::null()` for
this use case since it's not possible to construct directly anymore.

r? @emilio

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ports/geckolib/gecko_bindings/sugar/ns_style_coord.rs

@@ -3,9 +3,20 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use bindings::{Gecko_ResetStyleCoord, Gecko_SetStyleCoordCalcValue, Gecko_AddRefCalcArbitraryThread};
+use std::mem;
 use structs::{nsStyleCoord_Calc, nsStyleUnit, nsStyleUnion, nsStyleCoord, nsStyleSides, nsStyleCorners};
 use structs::{nsStyleCoord_CalcValue, nscoord};
 
+impl nsStyleCoord {
+    #[inline]
+    pub fn null() -> Self {
+        // Can't construct directly because it has private fields
+        let mut coord: Self = unsafe { mem::zeroed() };
+        coord.leaky_set_null();
+        coord
+    }
+}
+
 impl CoordData for nsStyleCoord {
     #[inline]
     fn unit(&self) -> nsStyleUnit {