Unify multiple sequential_promise_test definitions.

LayoutTests/http/tests/resources/testharness-helpers.js

Index: LayoutTests/http/tests/resources/testharness-helpers.js
diff --git a/LayoutTests/http/tests/resources/testharness-helpers.js b/LayoutTests/http/tests/resources/testharness-helpers.js
index 4d7af1ff9ca77c85696f35368ff3a7a1c4b2480b..b71b2494ec6d427d2608a506df2740bc77b23c45 100644
--- a/LayoutTests/http/tests/resources/testharness-helpers.js
+++ b/LayoutTests/http/tests/resources/testharness-helpers.js
@@ -161,3 +161,23 @@ function stringifyDOMObject(object)
     }
     return JSON.stringify(deepCopy(object));
 }
+
+(function() {
+  var promise_tests = Promise.resolve();
+  // Helper function to run promise tests one after the other.
+  // TODO(ortuno): Remove once https://github.com/w3c/testharness.js/pull/115/files
+  // gets through.
+  function sequential_promise_test(func, name) {
+    var test = async_test(name);
+    promise_tests = promise_tests.then(function() {
+      return test.step(func, test, test);
+    }).then(function() {
+      test.done();
+    }).catch(test.step_func(function(value) {
+      // step_func catches the error again so the error doesn't propagate.
+      throw value;
+    }));
+  }
+
+  self.sequential_promise_test = sequential_promise_test;
+})();