Revert "Create fallback config for UKM sampling."

This reverts commit 1e874ff9.

Reason for revert: This appears to be causing many UKM related tests to fail in official builds:
https://uberchromegw.corp.google.com/i/official.desktop.continuous/builders/win%20trunk/builds/86186

Original change's description:
> Create fallback config for UKM sampling.
> 
> Some clients do not have the sampling configuration for UKM records
> and the large number of entries can cause significant bias when weighted
> against those clients that are sampling.  Setting a large default
> sampling value will at least bias the other way.
> 
> Bug: 766909
> Change-Id: Ib6d656bd2ce4d82efb35281d09dc7b94128fadb2
> Reviewed-on: https://chromium-review.googlesource.com/1095596
> Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
> Commit-Queue: Brian White <bcwhite@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#566831}

TBR=rkaplow@chromium.org,asvitkine@chromium.org,bcwhite@chromium.org

Change-Id: I85b86053a3a41add9a5c93e31006b885b3b2f3d3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 766909
Reviewed-on: https://chromium-review.googlesource.com/1099617Reviewed-by: Reid Kleckner <rnk@chromium.org>
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566968}

chrome/browser/chrome_browser_field_trials.cc

@@ -28,9 +28,7 @@
 #include "chrome/common/chrome_switches.h"
 #include "components/metrics/metrics_pref_names.h"
 #include "components/metrics/persistent_system_profile.h"
-#include "components/ukm/ukm_recorder_impl.h"
 #include "components/variations/variations_associated_data.h"
-#include "components/version_info/version_info.h"
 
 #if defined(OS_ANDROID)
 #include "chrome/browser/chrome_browser_field_trials_mobile.h"
@@ -200,20 +198,22 @@ void InstantiatePersistentHistograms() {
 
 // Create a field trial to control metrics/crash sampling for Stable on
 // Windows/Android if no variations seed was applied.
-void CreateFallbackSamplingTrialIfNeeded(base::FeatureList* feature_list) {
+void CreateFallbackSamplingTrialIfNeeded(bool has_seed,
+                                         base::FeatureList* feature_list) {
 #if defined(OS_WIN) || defined(OS_ANDROID)
+  // Only create the fallback trial if there isn't already a variations seed
+  // being applied. This should occur during first run when first-run variations
+  // isn't supported. It's assumed that, if there is a seed, then it either
+  // contains the relavent study, or is intentionally omitted, so no fallback is
+  // needed.
+  if (has_seed)
+    return;
+
   ChromeMetricsServicesManagerClient::CreateFallbackSamplingTrial(
       chrome::GetChannel(), feature_list);
 #endif  // defined(OS_WIN) || defined(OS_ANDROID)
 }
 
-// Create a field trial to control UKM sampling for Stable if no variations
-// seed was applied.
-void CreateFallbackUkmSamplingTrialIfNeeded(base::FeatureList* feature_list) {
-  ukm::UkmRecorderImpl::CreateFallbackSamplingTrial(
-      chrome::GetChannel() == version_info::Channel::STABLE, feature_list);
-}
-
 }  // namespace
 
 ChromeBrowserFieldTrials::ChromeBrowserFieldTrials() {}
@@ -235,15 +235,7 @@ void ChromeBrowserFieldTrials::SetupFieldTrials() {
 void ChromeBrowserFieldTrials::SetupFeatureControllingFieldTrials(
     bool has_seed,
     base::FeatureList* feature_list) {
-  // Only create the fallback trials if there isn't already a variations seed
-  // being applied. This should occur during first run when first-run variations
-  // isn't supported. It's assumed that, if there is a seed, then it either
-  // contains the relavent studies, or is intentionally omitted, so no fallback
-  // is needed.
-  if (!has_seed) {
-    CreateFallbackSamplingTrialIfNeeded(feature_list);
-    CreateFallbackUkmSamplingTrialIfNeeded(feature_list);
-  }
+  CreateFallbackSamplingTrialIfNeeded(has_seed, feature_list);
 }
 
 void ChromeBrowserFieldTrials::InstantiateDynamicTrials() {

components/ukm/ukm_recorder_impl.cc

@@ -18,7 +18,6 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
 #include "components/ukm/ukm_source.h"
-#include "components/variations/variations_associated_data.h"
 #include "services/metrics/public/cpp/ukm_decode.h"
 #include "services/metrics/public/cpp/ukm_source_id.h"
 #include "third_party/metrics_proto/ukm/entry.pb.h"
@@ -170,43 +169,6 @@ bool HasUnknownMetrics(const ukm::builders::DecodeMap& decode_map,
 UkmRecorderImpl::UkmRecorderImpl() : recording_enabled_(false) {}
 UkmRecorderImpl::~UkmRecorderImpl() = default;
 
-// static
-void UkmRecorderImpl::CreateFallbackSamplingTrial(
-    bool is_stable_channel,
-    base::FeatureList* feature_list) {
-  static const char kSampledGroup_Stable[] = "Sampled_NoSeed_Stable";
-  static const char kSampledGroup_Other[] = "Sampled_NoSeed_Other";
-  const char* sampled_group = kSampledGroup_Other;
-  int default_sampling = 1;  // Sampling is 1-in-N; this is N.
-
-  // Nothing is sampled out except for "stable" which omits almost everything
-  // in this configuration. This is done so that clients that fail to receive
-  // a configuration from the server do not bias aggregated results because
-  // of a relatively large number of records from them.
-  if (is_stable_channel) {
-    sampled_group = kSampledGroup_Stable;
-    default_sampling = 1000000;
-  }
-
-  scoped_refptr<base::FieldTrial> trial(
-      base::FieldTrialList::FactoryGetFieldTrial(
-          kUkmSamplingRateFeature.name, 100, sampled_group,
-          base::FieldTrialList::kNoExpirationYear, 1, 1,
-          base::FieldTrial::ONE_TIME_RANDOMIZED, nullptr));
-
-  // Everybody (100%) should have a sampling configuration.
-  std::map<std::string, std::string> params = {
-      {"_default_sampling", base::IntToString(default_sampling)}};
-  variations::AssociateVariationParams(trial->trial_name(), sampled_group,
-                                       params);
-  trial->AppendGroup(sampled_group, 100);
-
-  // Setup the feature.
-  feature_list->RegisterFieldTrialOverride(
-      kUkmSamplingRateFeature.name, base::FeatureList::OVERRIDE_ENABLE_FEATURE,
-      trial.get());
-}
-
 void UkmRecorderImpl::SourceCounts::Reset() {
   *this = SourceCounts();
 }

components/ukm/ukm_recorder_impl.h

@@ -42,14 +42,6 @@ class UkmRecorderImpl : public UkmRecorder {
   UkmRecorderImpl();
   ~UkmRecorderImpl() override;
 
-  // Unconditionally attempts to create a field trial to control client side
-  // metrics/crash sampling to use as a fallback when one hasn't been
-  // provided. This is expected to occur on first-run on platforms that don't
-  // have first-run variations support. This should only be called when there is
-  // no existing field trial controlling the sampling feature.
-  static void CreateFallbackSamplingTrial(bool is_stable_channel,
-                                          base::FeatureList* feature_list);
-
   // Enables/disables recording control if data is allowed to be collected. The
   // |extensions| flag separately controls recording of chrome-extension://
   // URLs; this flag should reflect the "sync extensions" user setting.

testing/variations/fieldtrial_testing_config.json

@@ -4185,6 +4185,29 @@
             ]
         }
     ],
+    "UkmSamplingRate": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "ios",
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Sampled",
+                    "params": {
+                        "_default_sampling": "1"
+                    },
+                    "enable_features": [
+                        "UkmSamplingRate"
+                    ]
+                }
+            ]
+        }
+    ],
     "UpdateMenuItem": [
         {
             "platforms": [