Refactor getInstalledRelatedApps code and add manifest logic and tests.

third_party/WebKit/LayoutTests/installedapp/resources/installedapp-test-helper.js

+'use strict';
+
+function assert_relatedapplication_equals(actual, expected, description) {
+  assert_equals(actual.platform, expected.platform, description);
+  assert_equals(actual.url, expected.url, description);
+  assert_equals(actual.id, expected.id, description);
+}
+
+function assert_array_relatedapplication_equals(
+    actual, expected, description) {
+  assert_equals(actual.length, expected.length, description);
+
+  for (let i = 0; i < actual.length; i++)
+    assert_relatedapplication_equals(actual[i], expected[i], description);
+}