Don't try to create a process group when not root.

The WebView zygote doesn't run with uid 0 and so can't create a process
group, since the directory in the cgroupfs is only writable by root.
Skip this when the zygote isn't running with uid 0 to avoid the error
being printed, as this causes misattribution of bugs.

Bug: 62435375
Test: use WebView, check log
Change-Id: I50164bba5ac7587933d58ca0595336ba4fe33728