Disable the flaky NetworkQualityEstimator unittest on ios_simulator

NetworkQualityEstimator::TestEffectiveConnectionTypeObserver is disabled on
ios-simulator until it is clear why it is flaky.

BUG=641708

Review-Url: https://codereview.chromium.org/2559373005
Cr-Commit-Position: refs/heads/master@{#438722}

net/nqe/network_quality_estimator_unittest.cc

@@ -838,7 +838,14 @@ TEST(NetworkQualityEstimatorTest, DefaultHttpRTTBasedThresholds) {
 
 // Tests that |GetEffectiveConnectionType| returns correct connection type when
 // only transport RTT thresholds are specified in the variation params.
-TEST(NetworkQualityEstimatorTest, ObtainThresholdsOnlyTransportRTT) {
+#if defined(OS_IOS)
+// Flaky on iOS: crbug.com/672917.
+#define MAYBE_ObtainThresholdsOnlyTransportRTT \
+  DISABLED_ObtainThresholdsOnlyTransportRTT
+#else
+#define MAYBE_ObtainThresholdsOnlyTransportRTT ObtainThresholdsOnlyTransportRTT
+#endif
+TEST(NetworkQualityEstimatorTest, MAYBE_ObtainThresholdsOnlyTransportRTT) {
   std::map<std::string, std::string> variation_params;
   variation_params["effective_connection_type_algorithm"] =
       "TransportRTTOrDownstreamThroughput";