GitLab

Updates the mojo_core shared library so that it is actually suitable
for production IPC use. Namely:

- MojoInitialize now spins up a background thread for I/O and gives
  it to the core impl.
- A flag is added to Initialize(), allowing shared library consumers
  to initialize themselves as a broker process.
- The thunks helper correctly loads the mojo_core library with
  RTLD_DEEPBIND set, avoiding heap crossover within the core impl
  due to e.g. malloc and free lazy-binding to different heaps
- Fixes the thunks header to compile as C once again, as it was
  missing some struct keywords.
- Adds MojoShutdown to the core ABI as a necessary call for shared
  library consumers who want clean shutdown.

This also adds a new multiprocess test consuming the shared library
to tie together and validate all of the above changes.

Bug: 809320
Change-Id: Ic1c56d99c86fd4a2dc7f812ee152994ced35ece6
Reviewed-on: https://chromium-review.googlesource.com/c/1306347
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605109}