Make removing a transform not crash.

sky/tests/styles/transforms.sky

 <style>
   .container {
     height: 100px;
     width: 200px;
     margin: 30px;
     outline: 1px solid black;
   }
   .box {
     height: 100%;
     width: 100%;
@@ skipping 30 matching lines @@
       testTransformRoundTrip('translate(80px, 90px)', 'matrix(1, 0, 0, 1, 80, 90)');
     });
 
     test('percent translate should roundtrip', () {
       testTransformRoundTrip('translate(10px, 50%)', 'matrix(1, 0, 0, 1, 10, 60)');
     });
 
     test('scale should roundtrip', () {
       testTransformRoundTrip('scale(1.2, 0.8)', 'matrix(1.2, 0, 0, 0.8, 0, 0)');
     });
+
+    test('removing a transform should not crash', () {
+      testBox.style['transform'] = 'translate(0, 0, 0)';
+      testBox.getBoundingClientRect();
+      testBox.style.removeProperty('transform');
+      testBox.getBoundingClientRect();
+    });
   }
 </script>