headless: Expose child process command line to embedder

This allows embedders to adjust command line options when e.g. renderer
processes are forked.

Bug: 546953
Change-Id: Iac78013a8a002ce0dad7684159ddbfbbeee76321
Reviewed-on: https://chromium-review.googlesource.com/737874
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511781}
4 files changed