Track UKM discontinuities.

This allows checking for potential data quality issues.
See b/112853960

Change-Id: I4d361cbc18dde8589d6da23026bbfa8d4db547cb
Reviewed-on: https://chromium-review.googlesource.com/1182291
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590511}

components/ukm/ukm_recorder_impl.cc

@@ -239,6 +239,8 @@ void UkmRecorderImpl::EnableRecording(bool extensions) {
 
 void UkmRecorderImpl::DisableRecording() {
   DVLOG(1) << "UkmRecorderImpl::DisableRecording";
+  if (recording_enabled_)
+    recording_is_continuous_ = false;
   recording_enabled_ = false;
   extensions_enabled_ = false;
 }
@@ -250,6 +252,7 @@ void UkmRecorderImpl::DisableSamplingForTesting() {
 void UkmRecorderImpl::Purge() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   recordings_.Reset();
+  recording_is_continuous_ = false;
 }
 
 void UkmRecorderImpl::SetIsWebstoreExtensionCallback(
@@ -359,6 +362,9 @@ void UkmRecorderImpl::StoreRecordingsInReport(Report* report) {
   recordings_.entries.clear();
   recordings_.event_aggregations.clear();
 
+  report->set_is_continuous(recording_is_continuous_);
+  recording_is_continuous_ = true;
+
   // Keep at most |max_kept_sources|, prioritizing most-recent entries (by
   // creation time).
   const size_t max_kept_sources = GetMaxKeptSources();

components/ukm/ukm_recorder_impl.h

@@ -136,6 +136,9 @@ class UkmRecorderImpl : public UkmRecorder {
   // Indicates whether recording is enabled for extensions.
   bool extensions_enabled_ = false;
 
+  // Indicates whether recording continuity has been broken since last report.
+  bool recording_is_continuous_ = true;
+
   // Indicates if sampling has been enabled.
   bool sampling_enabled_ = true;