Bug 1182996 - Fix and add missing namespace comments. rs=ehsan

The bulk of this commit was generated by running:

  run-clang-tidy.py \
    -checks='-*,llvm-namespace-comment' \
    -header-filter=^/.../mozilla-central/.* \
    -fix

dom/plugins/ipc/PluginInstanceParent.cpp

@@ -1493,7 +1493,7 @@ ActorSearch(NPObject* aKey,
     return PL_DHASH_NEXT;
 }
 
-} // anonymous namespace
+} // namespace
 #endif // DEBUG
 
 bool