TaskScheduler: Add TaskTraits::WithWait().

base/task_scheduler/task_traits.h

Index: base/task_scheduler/task_traits.h
diff --git a/base/task_scheduler/task_traits.h b/base/task_scheduler/task_traits.h
index 93b6d71e9c7c3842ca9e65fcde9c66558d0face8..12ff6d3553daea9e5bbb951bf99730b300a1e952 100644
--- a/base/task_scheduler/task_traits.h
+++ b/base/task_scheduler/task_traits.h
@@ -89,18 +89,28 @@ class BASE_EXPORT TaskTraits {
   TaskTraits& operator=(const TaskTraits& other) = default;
   ~TaskTraits();
 
-  // Allows tasks with these traits to do file I/O.
+  // Allows tasks with these traits to wait on synchronous file I/O.
   TaskTraits& WithFileIO();
 
+  // Allows tasks with these traits to wait on things other than file I/O. In
+  // particular, they may wait on a WaitableEvent or a ConditionVariable, join a
+  // thread or a process, or make a blocking system call that doesn't involve
+  // interactions with the file system.
+  TaskTraits& WithWait();
+
   // Applies |priority| to tasks with these traits.
   TaskTraits& WithPriority(TaskPriority priority);
 
   // Applies |shutdown_behavior| to tasks with these traits.
   TaskTraits& WithShutdownBehavior(TaskShutdownBehavior shutdown_behavior);
 
-  // Returns true if file I/O is allowed by these traits.
+  // Returns true if waiting on synchronous file I/O is allowed by these traits.
   bool with_file_io() const { return with_file_io_; }
 
+  // Returns true if waiting on things other than file I/O is allowed by these
+  // traits.
+  bool with_wait() const { return with_wait_; }
+
   // Returns the priority of tasks with these traits.
   TaskPriority priority() const { return priority_; }
 
@@ -109,6 +119,7 @@ class BASE_EXPORT TaskTraits {
 
  private:
   bool with_file_io_;
+  bool with_wait_;
   TaskPriority priority_;
   TaskShutdownBehavior shutdown_behavior_;
 };