[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.
Merge pull request #2393 from jondong/naclLoading

[REG] Fixed NaCl loading issue introduced by rebase.