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/PluginModuleChild.cpp

@@ -67,7 +67,7 @@ const wchar_t * kMozillaWindowClass = L"MozillaWindowClass";
 namespace {
 // see PluginModuleChild::GetChrome()
 PluginModuleChild* gChromeInstance = nullptr;
-}
+} // namespace
 
 #ifdef MOZ_WIDGET_QT
 typedef void (*_gtk_init_fn)(int argc, char **argv);