The [Exposed] attribute is valid to use on partial interfaces per WebIDL:

https://heycam.github.io/webidl/#Exposed

We don't currently seem to consider it in the generated code. IDLCompiler.md says the following:

  >> Some extended attributes are transferred from the dependency interface
  >> definition to its members, because otherwise there is no way to apply the
  >> attributes to "just the members in this definition", due to merging. These
  >> are extended attributes for turning off a dependency interface, namely:
  >> `[Conditional], [PerContextEnabled],` and `[RuntimeEnabled].`

There are a few instances where a partial interfaces defined more restrictive exposure than the parent interface. I'm updating these in a CL, as *not* exposing them would have a bug, so in this case we got lucky.

https://codereview.chromium.org/2866023003/

I'm not aware of any users that require this behaviour for now, but we should track that we don't implement it.