Bug 1376873 - Make lock_block explicit; r=bwc

Differential Revision: https://phabricator.services.mozilla.com/D7468

--HG--
extra : rebase_source : c81c3a142595e24b4177b03c2fbf58b49ec0062d

media/mtransport/sigslot.h

@@ -212,7 +212,7 @@ class lock_block {
  public:
   mt_policy* m_mutex;
 
-  lock_block(mt_policy* mtx) : m_mutex(mtx) { m_mutex->lock(); }
+  explicit lock_block(mt_policy* mtx) : m_mutex(mtx) { m_mutex->lock(); }
 
   ~lock_block() { m_mutex->unlock(); }
 };