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

The blocking pool is deprecated. base/task_scheduler/post_task.h
should be used instead. More than 180 CLs to migrate the blocking
pool to base/task_scheduler/post_task.h have landed (ref. bug).

BUG=667892

Review-Url: https://codereview.chromium.org/2841173002
Cr-Commit-Position: refs/heads/master@{#468421}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 9d29982..2bb7036 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -319,6 +319,16 @@
       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, contact base/task_scheduler/OWNERS.',
+      ),
+      True,
+      (),
+    ),
 )