Separate the create and start phases in TaskSchedulerImpl.

base/task_scheduler/task_scheduler.h

Index: base/task_scheduler/task_scheduler.h
diff --git a/base/task_scheduler/task_scheduler.h b/base/task_scheduler/task_scheduler.h
index 9c7207ef0b550858941960135d39eb8a8baa1628..fa786c88efe7580a8193edb0ac0020fdb26e1c68 100644
--- a/base/task_scheduler/task_scheduler.h
+++ b/base/task_scheduler/task_scheduler.h
@@ -33,9 +33,17 @@ namespace base {
 class HistogramBase;
 
 // Interface for a task scheduler and static methods to manage the instance used
-// by the post_task.h API. Note: all base/task_scheduler users should go through
-// post_task.h instead of TaskScheduler except for the one callsite per process
-// which manages the process' instance.
+// by the post_task.h API.
+//
+// The task scheduler doesn't create threads before Start() is called. Tasks can
+// be posted before Start() is called, but they don't run until Start() is
+// called.
+//
+// The non-static methods of this class are thread-safe.

[robliao, 2017/04/24 22:32:19]

Nit: non-static -> instance

[fdoray, 2017/04/25 18:43:34]

Done.

+//
+// Note: all base/task_scheduler users should go through post_task.h instead of

[robliao, 2017/04/24 22:32:19]

Nit: all -> All

[gab, 2017/04/25 15:16:15]

I don't think this is required (and IMO capital letter here looks weird):
http://data.grammarbook.com/blog/commas/capitalization-with-colons/

"Capitalization rule with sentences after colons: If only one sentence follows
the colon, it is often not necessary to capitalize the first word of the new
sentence. If two or more sentences follow the colon, capitalize the first word
of each sentence following."

[robliao, 2017/04/25 17:37:28]

http://apvschicago.com/2011/04/capitalization-after-colons.html

Since the structure here is Note: [Complete Sentence and more], the sentence
should be capitalized.

[fdoray, 2017/04/25 18:43:34]

Done.

+// TaskScheduler except for the one callsite per process which manages the
+// process' instance.

[robliao, 2017/04/24 22:32:19]

Nit: process's

[gab, 2017/04/25 15:16:15]

Don't think this is required either (I sure always omit after 's' and have
committed many comments as such):
http://www.grammarbook.com/punctuation/apostro.asp

[robliao, 2017/04/25 17:37:27]

In this case, because process is singular, process's is the way to go. 
See
https://english.stackexchange.com/questions/1073/what-is-the-correct-possessi...

[fdoray, 2017/04/25 18:43:34]

Done.

 class BASE_EXPORT TaskScheduler {
  public:
   struct BASE_EXPORT InitParams {
@@ -59,6 +67,10 @@ class BASE_EXPORT TaskScheduler {
   // returned.
   virtual ~TaskScheduler() = default;
 
+  // Allows the task scheduler to create threads and run tasks following the
+  // |init_params| specification. CHECKs on failure.
+  virtual void Start(const InitParams& init_params) = 0;
+
   // Posts |task| with a |delay| and specific |traits|. |delay| can be zero.
   // For one off tasks that don't require a TaskRunner.
   virtual void PostDelayedTaskWithTraits(