Now that we have WR stood up with e10s, we should start unbreaking things that were broken as a result. In particular, there are various bits of code that send messages to the compositor process by using IPDL methods on PLayerTransaction (code doing this generally gets the ShadowLayerForwarder from the ClientLayerManager to use the IPDL bridge). In the WR branch we no longer have any PLayerTransaction instances being created, instead we have a PWebRenderBridge. So we should either copy the extra stuff from PLayerTransaction over to PWebRenderBridge or factor those things out to some other protocol (PCompositorBridge or some other sub-protocol) so that they can be used again. This shouldn't be done blindly - each protocol method in PLayerTransaction will need to be examined to determine whether we still need it in the WR world, and if so, how it needs to be adapted to WR.

I think this is pretty much done. Even if it isn't keeping this metabug open is pointless, we can just file specific bugs for IPDL messages we need to duplicate from PLayerTransaction to PWebRenderBridge.