Convert some non-debug logging on content/common to debug logging.

content/common/pepper_plugin_registry.cc

Index: content/common/pepper_plugin_registry.cc
===================================================================
--- content/common/pepper_plugin_registry.cc	(revision 107816)
+++ content/common/pepper_plugin_registry.cc	(working copy)
@@ -135,9 +135,10 @@
       std::string error;
       base::NativeLibrary library = base::LoadNativeLibrary(plugins[i].path,
                                                             &error);
-      LOG_IF(WARNING, !library) << "Unable to load plugin "
-                                << plugins[i].path.value() << " "
-                                << error;
+      DLOG_IF(WARNING, !library) << "Unable to load plugin "
+                                 << plugins[i].path.value() << " "
+                                 << error;
+      (void)library;  // Prevent release-mode warning.
     }
   }
 }