In https://chromium-review.googlesource.com/c/chromium/src/+/1093974 I attempted to make the code path for waiting on a non-child-process under Linux use NOTREACHED(), since if we hit this path then we cannot correctly reap the target process when it exits.  This change caused the android-marshmallow-arm64-rel
 bot to fail a number of content_browsertests.

We should diagnose why we're trying to wait for non-child processes on Android (possibly this is attempting to wait for renderer processes, launched via the Android Framework zygote, and hence not children of the browser?)