Bug 1101651 - Mark the constructor of WebRTC's ScopedXPCOM as explicit

Landed on a CLOSED TREE in order to fix build bustage.

media/mtransport/test/mtransport_test_utils.h

@@ -36,7 +36,7 @@
 #include "XPCOMRTInit.h"
 class ScopedXPCOM {
 public:
-  ScopedXPCOM(const char*)
+  explicit ScopedXPCOM(const char*)
   {
     NS_InitXPCOMRT();
   }