[REG] Fixed NaCl loading issue introduced by rebase.

According to the upstream change 01167110, the trusted plugin is
linked into the renderer directly instead of loading as a module
in runtime, and the ppGoogleNaClPlugin module has been removed;
so we need to change the plugin loading codes in
XWalkContentClient::AddPepperPlugins.
sure, though ppapi_permissions.h are not bound to NaCl, but as this file only uses with DISABLE_NACL macro here, we can include it inside as well.