Make much of the proxy thread-safe with 1 great big lock.

ppapi/proxy/plugin_resource_tracker.h

Index: ppapi/proxy/plugin_resource_tracker.h
diff --git a/ppapi/proxy/plugin_resource_tracker.h b/ppapi/proxy/plugin_resource_tracker.h
index 7268ddc3e5fd1bd6921e2edd8b6e3c4b84e6dbe2..30fafb2cd44f8059dccaa6fbe450f3f6ff3759f0 100644
--- a/ppapi/proxy/plugin_resource_tracker.h
+++ b/ppapi/proxy/plugin_resource_tracker.h
@@ -9,6 +9,7 @@
 #include <utility>
 
 #include "base/compiler_specific.h"
+#include "base/synchronization/lock.h"
 #include "ppapi/c/pp_completion_callback.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_stdint.h"
@@ -91,6 +92,9 @@ class PPAPI_PROXY_EXPORT PluginResourceTracker : public TrackerBase,
   typedef std::map<HostResource, PP_Resource> HostResourceMap;
   HostResourceMap host_resource_map_;
 
+  // The global lock for the plugin side of the proxy.
+  base::Lock proxy_lock_;
+
   DISALLOW_COPY_AND_ASSIGN(PluginResourceTracker);
 };