This appears to have started with bug 1345122: https://treeherder.mozilla.org/#/jobs?repo=autoland&fromchange=20109083820b6d5a79a37c04cfc42042c7d56087&noautoclassify&filter-searchStr=f1ea0499cc1d8823ecbf1764674e57dd56b00472&group_state=expanded&tochange=947da40a7edaa8881081e7fefe016f158a2b2b17
11 failures in 883 pushes (0.012 failures/push) were associated with this bug in the last 7 days. Repository breakdown: * autoland: 10 * mozilla-central: 1 Platform breakdown: * osx-10-10: 11 For more details, see: https://brasstacks.mozilla.com/orangefactor/?display=Bug&bugid=1360734&startday=2017-04-24&endday=2017-04-30&tree=all
It seems unlikely that the code for bug 1345122 caused this intermittent, seeing as how the code doesn't actually get called or initialized anywhere yet. I did a little bit of digging and found that the patch for bug 1360196 seems maybe more likely to have introduced the intermittent failure here, seeing as browser_net_open_request_in_tab.js is calling stuff that sounds related to bug 1360196.