Add IDL and initial Blink API for Background Sync

LayoutTests/http/tests/background_sync/resources/interfaces-worker.js

+importScripts('/resources/testharness.js');
+importScripts('/resources/testharness-helpers.js');
+
+test(function() {
+    assert_own_property(self, 'SyncManager', 'SyncManager needs to be exposed as a global.');
+    assert_will_be_idl_attribute(registration, 'syncManager', 'syncManager needs to be exposed on the registration.');
+
+    assert_inherits(registration.syncManager, 'register');
+    assert_inherits(registration.syncManager, 'getRegistration');
+    assert_inherits(registration.syncManager, 'getRegistrations');
+    // FIXME: Re-enable this once permissions are wired up
+    //assert_inherits(registration.syncManager, 'hasPermission');
+
+}, 'SyncManager should be exposed and have the expected interface.');
+
+test(function() {
+    assert_own_property(self, 'SyncRegistration', 'PushRegistration needs to be exposed as a global.');
+
+    // FIXME: Assert existence of the attributes when they are properly
+    // exposed in the prototype chain. https://crbug.com/43394
+
+    assert_own_property(SyncRegistration.prototype, 'unregister');
+
+}, 'SyncRegistration should be exposed and have the expected interface.');
+
+test(function() {
+    assert_own_property(self, 'SyncEvent');
+
+    var event = new SyncEvent('SyncEvent', {id: 'SyncEvent'});
+    assert_will_be_idl_attribute(event, 'registration');
+    assert_equals(event.type, 'SyncEvent');
+
+    // SyncEvent should be extending ExtendableEvent.
+    assert_inherits(event, 'waitUntil');
+
+}, 'SyncEvent should be exposed and have the expected interface.');
+