SELinux changes to accomodate starting the webview_zygote as a child of the zygote.

In this architecture, the system_server instructs the zygote to fork a
child-zygote to be the webview_zygote. The system_server tells this new
zygote to listen for fork requests on a random abstract unix socket of
its choosing.

A follow-up CL will remove the rules for starting webview_zygote via
init.

Bug: 63749735
Test: m
Test: Launch "Third-party licenses" activity from Settings, and it
      renders correctly via the WebView.
Merged-In: I864743943c11c18de386010ecd4b616721cb9954
Change-Id: I1c352e47b66eca3a3fa641daa6ecc3e7a889b54e
diff --git a/private/system_server.te b/private/system_server.te
index 752dee7..6e6212e 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -36,6 +36,7 @@
 # May kill zygote on crashes.
 allow system_server zygote:process sigkill;
 allow system_server crash_dump:process sigkill;
+allow system_server webview_zygote:process sigkill;
 
 # Read /system/bin/app_process.
 allow system_server zygote_exec:file r_file_perms;