While fixing Issue 652932, I noticed that the return value/error is non-deterministic when tapping a link that closes the Tab.  Before crrev.com/c/974791, we assumed that this behavior was affected only by whether or not the slim navigation manager is enabled, but I noticed that the behavior of these taps is flaky regardless of enabled flags or device vs. simulator.  This seems to be a race condition, where sometimes the tap finishes successfully, but other times the tap is handled on the JS side (causing the tab to close and web view to be invalidated) before the function returns.