Disable unit test NetworkContextTest.PreconnectMax

It's flaky on MacOS 10.10

TBR=lilyhoughton@chromium.org

Bug: 874419
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ib6cdc760ab0c6400eec0ba3a2e6901b079412722
Reviewed-on: https://chromium-review.googlesource.com/1175799
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583228}

services/network/network_context_unittest.cc

@@ -3230,7 +3230,8 @@ TEST_F(NetworkContextTest, PreconnectFour) {
   ASSERT_EQ(num_sockets, 4);
 }
 
-TEST_F(NetworkContextTest, PreconnectMax) {
+// Flaky; see http://crbug.com/874419
+TEST_F(NetworkContextTest, DISABLED_PreconnectMax) {
   std::unique_ptr<NetworkContext> network_context =
       CreateContextWithParams(CreateContextParams());