Add constexpr TaskTraits constructor.

base/task_scheduler/task_traits.cc

Index: base/task_scheduler/task_traits.cc
diff --git a/base/task_scheduler/task_traits.cc b/base/task_scheduler/task_traits.cc
index 4ebc64695c407772053ba0fbe7d61ffd77d435ce..326e2375adaf8d197b7aed5bc22e0cee7137b02d 100644
--- a/base/task_scheduler/task_traits.cc
+++ b/base/task_scheduler/task_traits.cc
@@ -12,17 +12,6 @@
 
 namespace base {
 
-// Do not rely on defaults hard-coded below beyond the guarantees described in
-// the header; anything else is subject to change. Tasks should explicitly
-// request defaults if the behavior is critical to the task.
-TaskTraits::TaskTraits()
-    : may_block_(false),
-      with_base_sync_primitives_(false),
-      priority_(TaskPriority::INHERITED),
-      shutdown_behavior_(TaskShutdownBehavior::SKIP_ON_SHUTDOWN) {}
-
-TaskTraits::~TaskTraits() = default;
-
 TaskTraits& TaskTraits::MayBlock() {
   may_block_ = true;
   return *this;