[cssom] Make an test assertion not show the number of CSS properties if it fails.

That causes unnecessary pain when updating tests across platforms, etc., and also unnecessary test failures when adding new properties for example.

Author: emilio

css/cssom/getComputedStyle-detached-subtree.html

@@ -17,7 +17,7 @@
     assert_true(!!element);
     let style = getComputedStyle(element);
     assert_true(!!style);
-    assert_equals(style.length, 0);
+    assert_true(style.length === 0);
     assert_equals(style.color, "");
   }, `getComputedStyle returns no style for ${description}`);
 }