How to add a new WebRender display item from a Gecko Display item, the general flow is to:

(1) Force layout to create a new active layer for the gecko display item.
(2) Plumb the data needed for the display item from content through WebRenderBridgeParent on the parent side.
(3) From WebRenderBridgeParent, call out into bindings.rs and implement the appropriate WR calls.

More detailed steps are:

1) Force layout to create an active layer for the gecko display item.
See http://searchfox.org/mozilla-central/rev/0f254a30d684796bcc8b6e2a102a0095d25842bb/layout/generic/nsTextFrame.cpp#4983
as an example for text layers. Ping Matt Woodrow or Markus Stange for help.

The Active layer part comes from nsDisplayText::GetLayerState

2) Create the new display item layer:

See text layer:
http://searchfox.org/mozilla-central/rev/0f254a30d684796bcc8b6e2a102a0095d25842bb/gfx/layers/Layers.h#2403

The layer should have all the information to display the item.

3) Create the WebRender equivalent layer. In YourLayerType::RenderLayer, serialize the data needed for the layer type.
4) If you need to add a custom IPC serialization mechanism, do it in WebRenderMessageUtils.h
5) Create a WebRender command to process the new layer type in WebRenderMessages.ipdlh. These are the struct OpDPPushYourLayerTypeHere
6) Add a new function in WebRender.h that will call out into webrender to render your display item.
7) In WebRenderBridgeParent::ProcessWebRenderCommands, call out to the new function in (6).
8) Fill out the function in (6) in bindings.rs to make webrender do the right thing. Generally, it's just push a display item.