Add a --no-zygote command line flag
This patch adds a --no-zygote command line flag which can be used to
completely disable the use of the zygote process. This is needed for
running Headless Chrome in environments where maintaining a forking
server such as the zygote is unnecessary and/or impractical.
Note that on Linux the sandbox needs the zygote to work, so --no-sandbox
must be used in conjunction with this new flag.