[testharness.js][docs] Improve and document assert_object_equals (web…

…-platform-tests#11642)

Fixes web-platform-tests#10262.

docs/_writing-tests/testharness-api.md

@@ -700,6 +700,11 @@ Relies on `===`, distinguishes between `-0` and `+0`, and has a specific check f
 asserts that `expected` is an Array, and `actual` is equal to one of the
 members i.e. `expected.indexOf(actual) != -1`
 
+### `assert_object_equals(actual, expected, description)`
+asserts that `actual` is an object and not null and that all enumerable
+properties on `actual` are own properties on `expected` with the same values,
+recursing if the value is an object and not null.
+
 ### `assert_array_equals(actual, expected, description)`
 asserts that `actual` and `expected` have the same
 length and the value of each indexed property in `actual` is the strictly equal

resources/testharness.js

@@ -997,6 +997,9 @@ policies and contribution forms [3].
 
     function assert_object_equals(actual, expected, description)
     {
+         assert(typeof actual === "object" && actual !== null, "assert_object_equals", description,
+                                                               "value is ${actual}, expected object",
+                                                               {actual: actual});
          //This needs to be improved a great deal
          function check_equal(actual, expected, stack)
          {