Allow zygote to keep socket to heapprofd.

This allows users to target zygote for heap profiling without
causing it to crash on ForkCommon.

This also allows us to profile zygote in benchmark runs where we want to
profile as many processes as possible.

Bug: 117821125

Test: m
Test: flashed walleye

Change-Id: I3f41b64e0a64e362eafd02cd4e86cb77e434d94c