Add presubmit check against usage of BrowserThread::GetBlockingPool().

PRESUBMIT.py

 # 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.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into depot_tools.
 """
 
@@ skipping 301 matching lines @@
       ),
     ),
     (
       '#pragma comment(lib,',
       (
         'Specify libraries to link with in build files and not in the source.',
       ),
       True,
       (),
     ),
+    (
+      'BrowserThread::GetBlockingPool',
+      (
+        'Use base/task_scheduler/post_task.h instead of the blocking pool. See',
+        'mapping between both APIs in content/public/browser/browser_thread.h.',
+        'For questions, reach out to base/task_scheduler/OWNERS.',

[robliao, 2017/05/01 18:03:01]

Nit: s/reach out to/contact/

[fdoray, 2017/05/01 18:05:22]

Done.

+      ),
+      True,
+      (),
+    ),
 )
 
 
 _IPC_ENUM_TRAITS_DEPRECATED = (
     'You are using IPC_ENUM_TRAITS() in your code. It has been deprecated.\n'
     'See http://www.chromium.org/Home/chromium-security/education/security-tips-for-ipc')
 
 
 _VALID_OS_MACROS = (
     # Please keep sorted.
@@ skipping 2122 matching lines @@
       output_api,
       json_url='http://chromium-status.appspot.com/current?format=json'))
 
   results.extend(
       input_api.canned_checks.CheckPatchFormatted(input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasBugField(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   return results