Hydra update issue when removing parent primvar

[marktucker]

Description of Issue

When a primvar is inherited from a parent to a child prim, and the primvar on the parent prim is blocked (or deleted), the child prim is not updated in hydra to reflect the change.

Steps to Reproduce

1. Load grid.usda into usdview from the attached archive.
   grid.zip
2. In the Interpreter window, run:
   l = Sdf.Layer.FindOrOpen("gridnocolor.usda")
   usdviewApi.stage.GetRootLayer().TransferContent(l)
3. The color on the grid should disappear, but it does not. This happens with all the render delegates I tested.

Package Versions

19.11-rc1

[jilliene]

Filed as internal issue #USD-5639