Revert "predictors: Enable the new preconnect predictor by default"

This reverts commit 5f1039f1.

Reason for revert: multiple tests on Chrome OS became flaky because of disk I/O errors.

Original change's description:
> predictors: Enable the new preconnect predictor by default
> 
> The old predictor is automatically disabled when the new one is enabled.
> 
> Change-Id: Ia5c1a79dbeb0e096720fc3d50da3dc707b3761a6
> Reviewed-on: https://chromium-review.googlesource.com/1042388
> Reviewed-by: Egor Pasko <pasko@chromium.org>
> Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#555766}

TBR=pasko@chromium.org,alexilin@chromium.org
Bug: 839886

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I8c286f29cee249990c57486851842e4d9dd900ac
Reviewed-on: https://chromium-review.googlesource.com/1044326Reviewed-by: Alexandr Ilin <alexilin@chromium.org>
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556130}

chrome/browser/predictors/loading_predictor_config.cc

@@ -39,7 +39,7 @@ const char kPreconnectMode[] = "preconnect";
 const char kNoPreconnectMode[] = "no-preconnect";
 
 const base::Feature kSpeculativePreconnectFeature{
-    kSpeculativePreconnectFeatureName, base::FEATURE_ENABLED_BY_DEFAULT};
+    kSpeculativePreconnectFeatureName, base::FEATURE_DISABLED_BY_DEFAULT};
 
 bool MaybeEnableSpeculativePreconnect(LoadingPredictorConfig* config) {
   if (!base::FeatureList::IsEnabled(kSpeculativePreconnectFeature))
@@ -47,9 +47,6 @@ bool MaybeEnableSpeculativePreconnect(LoadingPredictorConfig* config) {
 
   std::string mode_value = base::GetFieldTrialParamValueByFeature(
       kSpeculativePreconnectFeature, kModeParamName);
-  if (mode_value.empty())
-    mode_value = kPreconnectMode;
-
   if (mode_value == kLearningMode) {
     if (config) {
       config->mode |= LoadingPredictorConfig::LEARNING;

chrome/browser/predictors/loading_predictor_config_unittest.cc

@@ -66,14 +66,12 @@ class LoadingPredictorConfigTest : public testing::Test {
 LoadingPredictorConfigTest::LoadingPredictorConfigTest()
     : profile_(new TestingProfile()) {}
 
-TEST_F(LoadingPredictorConfigTest, IsEnabledByDefault) {
+TEST_F(LoadingPredictorConfigTest, IsDisabledByDefault) {
   LoadingPredictorConfig config;
-  EXPECT_TRUE(MaybeEnableSpeculativePreconnect(&config));
+  EXPECT_FALSE(MaybeEnableSpeculativePreconnect(&config));
 
-  EXPECT_TRUE(config.IsLearningEnabled());
-  EXPECT_TRUE(config.should_disable_other_preconnects);
-  EXPECT_FALSE(IsNetPredictorEnabled());
-  EXPECT_TRUE(config.IsPreconnectEnabledForSomeOrigin(profile_.get()));
+  EXPECT_FALSE(config.IsLearningEnabled());
+  EXPECT_FALSE(config.IsPreconnectEnabledForSomeOrigin(profile_.get()));
 }
 
 TEST_F(LoadingPredictorConfigTest, EnablePreconnectLearning) {