Allow WebExternalTextureLayerClient to work with mailboxes.

webkit/plugins/ppapi/ppapi_plugin_instance.cc

Index: webkit/plugins/ppapi/ppapi_plugin_instance.cc
diff --git a/webkit/plugins/ppapi/ppapi_plugin_instance.cc b/webkit/plugins/ppapi/ppapi_plugin_instance.cc
index a46d4d2ee214cc5eb1e6b94b8dce36653f535e19..08d9af336b750851e8db4342731130469da17126 100644
--- a/webkit/plugins/ppapi/ppapi_plugin_instance.cc
+++ b/webkit/plugins/ppapi/ppapi_plugin_instance.cc
@@ -2106,6 +2106,10 @@ WebKit::WebGraphicsContext3D* PluginInstance::Context3d() {
   return bound_graphics_3d_->platform_context()->GetParentContext();
 }
 
+bool PluginInstance::PrepareTextureMailbox(cc::TextureMailbox* out_mailbox) {
+  return false;
+}
+
 void PluginInstance::NumberOfFindResultsChanged(PP_Instance instance,
                                                 int32_t total,
                                                 PP_Bool final_result) {