Expose durable storage to workers.

LayoutTests/http/tests/storage/resources/durability-worker.js

+importScripts("/resources/testharness.js");
+
+test(function() { assert_true("storage" in navigator); },
+    "These worker tests require navigator.storage");
+
+test(function() { assert_false("requestPersistent" in navigator.storage); },

[jsbell, 2015/08/21 19:05:28]

On the TODO list (but low priority): we should add idlharness tests

+    "navigator.storage.requestPersistent should not exist in workers");
+
+promise_test(function() {
+    var promise = navigator.storage.persistentPermission();
+    assert_true(promise instanceof Promise,
+        "navigator.storage.persistentPermission() returned a Promise.")
+    return promise.then(function (result) {
+        // Layout tests get canned results, not the value per spec. So testing
+        // their values here would only be testing our test plumbing. But we can
+        // test that the type of the returned value is correct.
+        assert_equals(typeof result, "string", result + " should be a string");
+        assert_greater_than(result.length, 0, "result should have length >0");
+    });
+}, "navigator.storage.persistentPermission returns a promise that resolves.");
+
+done();