Exempt the Service Thread from BLOCK_SHUTDOWN DCHECKs

base/threading/thread.h

 // Copyright (c) 2012 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_THREADING_THREAD_H_
 #define BASE_THREADING_THREAD_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 227 matching lines @@
 
   // Returns the thread ID.  Should not be called before the first Start*()
   // call.  Keeps on returning the same ID even after a Stop() call. The next
   // Start*() call renews the ID.
   //
   // WARNING: This function will block if the thread hasn't started yet.
   //
   // This method is thread-safe.
   PlatformThreadId GetThreadId() const;
 
+  // Returns the current thread handle. If called before Start*() returns or
+  // after Stop() returns, an empty thread handle will be returned.
+  //
+  // This method is thread-safe.

[gab, 2017/05/04 18:11:58]

TODO(robliao): remove this when it no longer needs to be temporarily exposed for
http://crbug.com/717380.

...so its usage doesn't spread like wildfire and we can actually remove it when
we fix the current use case.

[robliao, 2017/05/04 19:01:49]

Done.

+  PlatformThreadHandle GetThreadHandle() const;
+
   // Returns true if the thread has been started, and not yet stopped.
   bool IsRunning() const;
 
  protected:
   // Called just prior to starting the message loop
   virtual void Init() {}
 
   // Called to start the run loop
   virtual void Run(RunLoop* run_loop);
 
@@ skipping 77 matching lines @@
   // This class is not thread-safe, use this to verify access from the owning
   // sequence of the Thread.
   SequenceChecker owning_sequence_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_H_