Splitting off from issue 593720, to implement the long term solution.

Context:

Chrome on Android currently reads command line flags from /data/local/tmp, but this is not allowed by SELinux policy from N and above.

torne@:

> Long-term, what we should do is actually store this information in the application's own data directory instead of a global location, because the SELinux policy is pretty much explicitly intended to prevent there being easily-writable global state on user builds which apps can access and maybe trust inappropriately. This would make it hard to set the command line on an unrooted device, though; not sure how we'd handle that.

torne, can you help triaging this?