Removing the exported symbols is straightforward enough.
The only wrinkle is that PeerConnectionCtx.cpp gets compiled with and
without MOZILLA_INTERNAL_API. When compiling without
MOZILLA_INTERNAL_API, mozilla::services::Get*Service was redirected to
the _external_* symbol variants. But as the _external_* symbols no
longer exist, PeerConnectionCtx.cpp's code no longer worked.
Fortunately, PeerConnectionCtx.cpp already contains a few #ifdef
MOZILLA_INTERNAL_API blocks to handle internal/external compilation;
fixing this newest issue was just a matter of extending existing blocks
and adding new ones. The key observation is that we never added any
observers when compiling without MOZILLA_INTERNAL_API, so we can #ifdef
out the removals of observers, as those would have no effect, and simply
skip getting the observer service for the observer additions if we're
compiling without MOZILLA_INTERNAL_API.
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