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.

BUG=546953

Review-Url: https://codereview.chromium.org/2384163002
Cr-Commit-Position: refs/heads/master@{#423152}
4 files changed