Make it possible to have 1 PpapiGlobals per thread. Update unit tests.

ppapi/proxy/plugin_var_serialization_rules.cc

Index: ppapi/proxy/plugin_var_serialization_rules.cc
diff --git a/ppapi/proxy/plugin_var_serialization_rules.cc b/ppapi/proxy/plugin_var_serialization_rules.cc
index a4eea5b78d2fa5d050114dd0524e8ece483b5932..940ff141bc3756a543ff64bc9884d964e2aabafa 100644
--- a/ppapi/proxy/plugin_var_serialization_rules.cc
+++ b/ppapi/proxy/plugin_var_serialization_rules.cc
@@ -85,7 +85,7 @@ PP_Var PluginVarSerializationRules::ReceivePassRef(const PP_Var& var,
   // plugin code started to return a value, which means it gets another ref
   // on behalf of the caller. This needs to be transferred to the plugin and
   // folded in to its set of refs it maintains (with one ref representing all
-  // fo them in the browser).
+  // of them in the browser).
   if (var.type == PP_VARTYPE_OBJECT) {
     DCHECK(dispatcher->IsPlugin());
     return var_tracker_->ReceiveObjectPassRef(