Add message from grandchild to parent to indicate successful host launch.

Previously, the grandparent process would assume the host started successfully
if the grandchild closed the pipe. However, this happens regardless of the
exit code of the grandchild process. This CL allows commands to be sent between
the processes in addition to log messages, and defines an explicit command to
indicate successful start-up; if the grandparent does not receive this command,
it indicates failure via a non-zero exit code.

BUG=332930
TEST=chmod -x remoting_me2me_host; linux_me2me_host.py --start; echo $? (should give 1)

Review URL: https://codereview.chromium.org/1685793003

Cr-Commit-Position: refs/heads/master@{#375680}
1 file changed