Fix MallocSizeOf for TypedSize2D.

TypedSize2D's MallocSizeOf impl has two problems.

- It measures `width` twice, and `height` not at all.

- It erroneously asserts that `width` and `height` are scalars. This
  seems reasonable at first blush, but Stylo uses
  `BorderRadius<LengthAndPercentage>` which contains a
  `TypedSize2D<LengthAndPercentage, UnknownUnit>`, and
  `LengthAndPercentage` is non-scalar.

This patch fixes both of these problems, and also removes a low-value
`use` statement.

components/malloc_size_of/lib.rs

@@ -64,7 +64,6 @@ extern crate servo_arc;
 extern crate smallbitvec;
 extern crate smallvec;
 
-use euclid::TypedSize2D;
 use servo_arc::Arc;
 use smallvec::{Array, SmallVec};
 use std::hash::{BuildHasher, Hash};
@@ -391,11 +390,9 @@ impl MallocSizeOf for smallbitvec::SmallBitVec {
     }
 }
 
-impl<T: MallocSizeOf, U> MallocSizeOf for TypedSize2D<T, U> {
+impl<T: MallocSizeOf, U> MallocSizeOf for euclid::TypedSize2D<T, U> {
     fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
-        let n = self.width.size_of(ops) + self.width.size_of(ops);
-        assert!(n == 0);    // It would be very strange to have a non-zero value here...
-        n
+        self.width.size_of(ops) + self.height.size_of(ops)
     }
 }