ScopedIPCSupport has two separate and subtly different implementations:
mojo::edk::test::ScopedIPCSupport for test-only use, and
mojo::edk::ScopedIPCSupport for general-purpose use.

This unifies them into a much simpler general-purpose ScopedIPCSupport
implementation suitable for all practical environments.

Also removes mojo::edk::ProcessDelegate, as it is abstraction overkill.
Instead of InitIPCSupport taking a ProcessDelegate which goes unused
until shutdown, ShutdownIPCSupport now simply takes a callback to be run
upon completion.

BUG=None
R=jam@chromium.org
TBR=haraken@chromium.org

Review-Url: https://codereview.chromium.org/2620633004
Cr-Commit-Position: refs/heads/master@{#443539}