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 300 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.',
+      ),
+      True,

[gab, 2017/04/28 01:58:24]

Does True here mean "block CQ" or "warning only"? I think this should still be
warning only as code that needs to be in sync with yet to be migrated code can
still use the BlockingPool legitimately if only to be on the right sequence
(that should be mentioned in the presubmit comment as well)

[fdoray, 2017/05/01 17:39:03]

Discussed offline.

If a components already has code to get a SequencedTaskRunner from
BrowserThread::GetBlockingPool(), this check won't prevent it from posting new
tasks to it.

If a component want to get a new SequencedTaskRunner or wants to post a parallel
tasks, it should use the base/task_scheduler/post_task.h API.

+      (),
+    ),
 )
 
 
 _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 2119 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