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

ppapi/proxy/plugin_resource_tracker.cc

Index: ppapi/proxy/plugin_resource_tracker.cc
diff --git a/ppapi/proxy/plugin_resource_tracker.cc b/ppapi/proxy/plugin_resource_tracker.cc
index 3fee320979f3eed0497f05b5333054680a795f5c..1616eda4c37502624e457bb2a5f24073e5ac0642 100644
--- a/ppapi/proxy/plugin_resource_tracker.cc
+++ b/ppapi/proxy/plugin_resource_tracker.cc
@@ -18,17 +18,9 @@ namespace ppapi {
 namespace proxy {
 
 PluginResourceTracker::PluginResourceTracker() {
-#ifdef ENABLE_PEPPER_THREADING
-  // Set the global proxy lock, since the plugin-side of the proxy needs to be
-  // synchronized.
-  ppapi::ProxyLock::Set(&proxy_lock_);
-#endif
 }
 
 PluginResourceTracker::~PluginResourceTracker() {
-#ifdef ENABLE_PEPPER_THREADING
-  ppapi::ProxyLock::Reset();
-#endif
 }
 
 PP_Resource PluginResourceTracker::PluginResourceForHostResource(