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

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 819bf7337311411589c2bfb4c872465e76bd1e58..8901195c80653a3280b30269f1caa68452d21c04 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -318,6 +318,14 @@ _BANNED_CPP_FUNCTIONS = (
       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.

+      (),
+    ),
 )