TaskScheduler: Add TaskTraits::WithWait().

base/task_scheduler/task_traits.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 #define BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 
 #include <stdint.h>
 
 #include <iosfwd>
@@ skipping 71 matching lines @@
   //     (1) no I/O,
   //     (2) low priority, and
   //     (3) may block shutdown or be skipped on shutdown.
   // Tasks that require stricter guarantees should highlight those by requesting
   // explicit traits below.
   TaskTraits();
   TaskTraits(const TaskTraits& other) = default;
   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_; }
 
   // Returns the shutdown behavior of tasks with these traits.
   TaskShutdownBehavior shutdown_behavior() const { return shutdown_behavior_; }
 
  private:
   bool with_file_io_;
+  bool with_wait_;
   TaskPriority priority_;
   TaskShutdownBehavior shutdown_behavior_;
 };
 
 // Returns string literals for the enums defined in this file. These methods
 // should only be used for tracing and debugging.
 BASE_EXPORT const char* TaskPriorityToString(TaskPriority task_priority);
 BASE_EXPORT const char* TaskShutdownBehaviorToString(
     TaskShutdownBehavior task_priority);
 
 // Stream operators so that the enums defined in this file can be used in
 // DCHECK and EXPECT statements.
 BASE_EXPORT std::ostream& operator<<(std::ostream& os,
                                      const TaskPriority& shutdown_behavior);
 BASE_EXPORT std::ostream& operator<<(
     std::ostream& os,
     const TaskShutdownBehavior& shutdown_behavior);
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRAITS_H_