Introduce TaskRunner::RunsTasksInCurrentSequence()

base/task_runner.h

Index: base/task_runner.h
diff --git a/base/task_runner.h b/base/task_runner.h
index 0421d564e6d75cdc8f7d763ba5c95a3b37169067..d1ea2391712b99379d8589df0bff89b1eda38280 100644
--- a/base/task_runner.h
+++ b/base/task_runner.h
@@ -78,7 +78,22 @@ class BASE_EXPORT TaskRunner
   //
   // It is valid for an implementation to always return true, or in
   // general to use 'true' as a default value.

[gab, 2017/04/18 15:00:32]

//
// Drepecated: favor RunsTasksInCurrentSequence().
// TODO(http://crbug.com/665062): mass redirect callers and remove this.

-  virtual bool RunsTasksOnCurrentThread() const = 0;
+  bool RunsTasksOnCurrentThread() const;
+
+  // Returns true iff tasks posted to this TaskRunner are sequenced
+  // with this call.
+  //
+  // In particular:
+  // - Returns true if this is a SequencedTaskRunner to which the
+  //   current task was posted.
+  // - Returns true if this is a SequencedTaskRunner bound to the
+  //   same sequence as the SequencedTaskRunner to which the current
+  //   task was posted.
+  // - Returns true if this is a SingleThreadTaskRunner bound to
+  //   the current thread.
+  // - Returns false if this is a TaskRunner that can run its tasks
+  //   in parallel.

[gab, 2017/04/18 15:00:32]

This last statement isn't true yet (SchedulerParallelTaskRunner and
SequencedWorkerPool::Inner). We'd like it to be but it doesn't need to block
initiating this migration. Replace it with:

// TODO(http://crbug.com/665062): this API doesn't make sense for parallel
TaskRunners. Introduce alternate static APIs for documentation purposes of "this
runs in pool X", have RunsTasksInCurrentSequence() return false for parallel
TaskRunners, and ultimately move this method down to SequencedTaskRunner.

+  virtual bool RunsTasksInCurrentSequence() const = 0;
 
   // Posts |task| on the current TaskRunner.  On completion, |reply|
   // is posted to the thread that called PostTaskAndReply().  Both