Fix for flacky tests

The device/emulator may return different signal strengths, but the
tests are designed for constant strengths. When receiving different
signal strength tests are failing.
Made the signal strength in the tests to be constant.
In particular, these tests:
NetworkQualitiesPrefManager.WriteAndReadWithMultipleNetworkIDs
NetworkQualityEstimatorTest.CacheObserver
NetworkQualityEstimatorTest.TestTCPSocketRTT

R=tbansal@chromium.org, ryansturm@chromium.org
Bug:
TEST=tests was failing on emulator
"system-images;android-25;google_apis;x86"

Change-Id: I6fd1b4da633218659b6f4558634c97a86e336a26
Reviewed-on: https://chromium-review.googlesource.com/c/1341543Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609471}

net/nqe/network_quality_estimator_test_util.cc

@@ -348,6 +348,10 @@ nqe::internal::NetworkID TestNetworkQualityEstimator::GetCurrentNetworkID()
                                   INT32_MIN);
 }
 
+int32_t TestNetworkQualityEstimator::GetCurrentSignalStrength() const {
+  return INT32_MIN;
+}
+
 TestNetworkQualityEstimator::LocalHttpTestServer::LocalHttpTestServer(
     const base::FilePath& document_root) {
   AddDefaultHandlers(document_root);

net/nqe/network_quality_estimator_test_util.h

@@ -243,9 +243,9 @@ class TestNetworkQualityEstimator : public NetworkQualityEstimator {
                              base::TimeDelta rtt) override;
 
   // NetworkQualityEstimator implementation that returns the overridden
-  // network
-  // id (instead of invoking platform APIs).
+  // network id and signal strength (instead of invoking platform APIs).
   nqe::internal::NetworkID GetCurrentNetworkID() const override;
+  int32_t GetCurrentSignalStrength() const override;
 
   // Net log provided to network quality estimator.
   std::unique_ptr<net::BoundTestNetLog> net_log_;