Mojo Channel perf tests use an EDK supplied helper for multiprocess
client setup. This helper assumes ownership of the client's
primordial message pipe. IPC perf tests take ownership of the pipe
and pass it to an IPC::Channel. These are conflicting policies.

This allows a test helper user to indicate that the helper code should
not assume ownership of its pipe handle.

BUG=677202
R=yzshen@chromium.org

Review-Url: https://codereview.chromium.org/2766763002
Cr-Commit-Position: refs/heads/master@{#458497}