Added a ResourceMessageFilter for handling resource messages on another thread.

ppapi/ppapi_host.gypi

Index: ppapi/ppapi_host.gypi
diff --git a/ppapi/ppapi_host.gypi b/ppapi/ppapi_host.gypi
index 8967e456ab595f7cff43244812c0f05befe37a8b..2c05afe6d880c876ee3b2997ae39d7447f088d32 100644
--- a/ppapi/ppapi_host.gypi
+++ b/ppapi/ppapi_host.gypi
@@ -21,6 +21,8 @@
         'PPAPI_HOST_IMPLEMENTATION',
       ],
       'sources': [
+        'host/background_thread_message_filter.cc',
+        'host/background_thread_message_filter.h',
         'host/dispatch_host_message.h',
         'host/host_factory.h',
         'host/host_message_context.cc',
@@ -32,6 +34,8 @@
         'host/ppapi_host_export.h',
         'host/resource_host.cc',
         'host/resource_host.h',
+        'host/resource_host_message_filter.cc',
+        'host/resource_host_message_filter.h',
       ],
     },
   ],