Macros and {} variadic template don't play well together.

{} initialization of LAZY_TASK_RUNNERs doesn't work when providing
multiple traits. Update documentation to suggest explicit TaskTraits
initialization.

R=robliao@chromium.org

Bug: 730170
Change-Id: Icbd93f47f9d3f36c344ea892ed4b7e11b00becd3
Reviewed-on: https://chromium-review.googlesource.com/580531Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488676}

base/task_scheduler/lazy_task_runner.h

@@ -38,7 +38,9 @@
 //
 // namespace {
 // base::LazySequencedTaskRunner g_sequenced_task_runner =
-//     LAZY_SEQUENCED_TASK_RUNNER_INITIALIZER({base::MayBlock()});
+//     LAZY_SEQUENCED_TASK_RUNNER_INITIALIZER(
+//         base::TaskTraits(base::MayBlock(),
+//                          base::TaskPriority::USER_VISIBLE));
 // }  // namespace
 //
 // void SequencedFunction() {