Allow file read/write to darwin-user-temp-dir in network.sb.

Bug: 1004691
Change-Id: Iba2a3a59708a7d6ef62ea73087475825fe8d4c34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1912888Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715771}

services/service_manager/sandbox/mac/network.sb

@@ -51,6 +51,7 @@
 )
 (allow file-read* file-write*
   (subpath (param darwin-user-cache-dir))
+  (subpath (param darwin-user-temp-dir))
 )
 (if (param-defined? network-service-test-certs-dir)
   (allow file-read* (subpath (param network-service-test-certs-dir))))