Switch to new memory metrics collection timing.

Previously, memory metrics were collected at fixed intervals. The new timing
uses a poisson process to emit the same number of events, but at randomly chosen
times.

Bug: 895474
Change-Id: I3e9564372e7b0c71f58965e6b238efe1c1de2ae9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836005
Auto-Submit: Erik Chen <erikchen@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702993}

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

@@ -570,8 +570,7 @@ void ChromeBrowserMainExtraPartsMetrics::PreBrowserStart() {
 }
 
 void ChromeBrowserMainExtraPartsMetrics::PostBrowserStart() {
-  if (!base::FeatureList::IsEnabled(kMemoryMetricsOldTiming))
-    RecordMemoryMetricsAfterDelay();
+  RecordMemoryMetricsAfterDelay();
   RecordLinuxGlibcVersion();
 #if defined(USE_X11)
   UMA_HISTOGRAM_ENUMERATION("Linux.WindowManager", GetLinuxWindowManager(),

chrome/browser/metrics/chrome_metrics_service_client.cc

@@ -45,7 +45,6 @@
 #include "chrome/browser/metrics/https_engagement_metrics_provider.h"
 #include "chrome/browser/metrics/metrics_reporting_state.h"
 #include "chrome/browser/metrics/network_quality_estimator_provider_impl.h"
-#include "chrome/browser/metrics/process_memory_metrics_emitter.h"
 #include "chrome/browser/metrics/sampling_metrics_provider.h"
 #include "chrome/browser/metrics/subprocess_metrics_provider.h"
 #include "chrome/browser/profiles/profile_manager.h"
@@ -811,12 +810,6 @@ void ChromeMetricsServiceClient::CollectFinalHistograms() {
   scoped_refptr<MetricsMemoryDetails> details(
       new MetricsMemoryDetails(callback));
   details->StartFetch();
-
-  if (base::FeatureList::IsEnabled(kMemoryMetricsOldTiming)) {
-    scoped_refptr<ProcessMemoryMetricsEmitter> emitter(
-        new ProcessMemoryMetricsEmitter);
-    emitter->FetchAndEmitProcessMemoryMetrics();
-  }
 }
 
 void ChromeMetricsServiceClient::OnMemoryDetailCollectionDone() {

chrome/browser/metrics/process_memory_metrics_emitter.cc

@@ -47,9 +47,6 @@ using memory_instrumentation::HistogramProcessTypeToString;
 using memory_instrumentation::kMemoryHistogramPrefix;
 using ukm::builders::Memory_Experimental;
 
-const base::Feature kMemoryMetricsOldTiming{"MemoryMetricsOldTiming",
-                                            base::FEATURE_ENABLED_BY_DEFAULT};
-
 namespace {
 
 const char kEffectiveSize[] = "effective_size";

chrome/browser/metrics/process_memory_metrics_emitter.h

@@ -9,7 +9,6 @@
 
 #include "base/callback.h"
 #include "base/containers/flat_map.h"
-#include "base/feature_list.h"
 #include "base/memory/ref_counted.h"
 #include "base/optional.h"
 #include "base/process/process_handle.h"
@@ -26,8 +25,6 @@ namespace performance_manager {
 class Graph;
 }
 
-extern const base::Feature kMemoryMetricsOldTiming;
-
 // This class asynchronously fetches memory metrics for each process, and then
 // emits UMA metrics from those metrics.
 // Each instance is self-owned, and will delete itself once it has finished