Add comments explaining different results for...

Add comments explaining different results for NetworkContextTest.ProxyErrorClientNotifiedOfPacError on iOS.

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Iee8707645813ad7af92f7a4e84403ccacc22abd0
Reviewed-on: https://chromium-review.googlesource.com/1208932
Commit-Queue: Eric Roman <eroman@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589349}

services/network/network_context_unittest.cc

@@ -3748,7 +3748,8 @@ TEST_F(NetworkContextTest, ProxyErrorClientNotifiedOfPacError) {
 
   EXPECT_EQ(0u, request_errors.size());
 
-// TODO(eroman): Why is this failing on iOS?
+// TODO(https://crbug.com/881124): Mocking proxy resolution doesn't work
+//                                 on iOS.
 #if defined(OS_IOS)
   if (pac_errors.empty())
     return;