[legacymetrics] Fix use-after-free in LegacyMetricsClient.

Handles the crasher in which the metrics recorder channel is
disconnected while additional metrics are being gathered.

Test: cr_fuchsia_base_unittests
Bug: 1147146
Change-Id: Iff987933233bd1565ed052b1ca1e1225b30e782d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538331
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828845}

fuchsia/base/legacymetrics_client.cc

@@ -85,9 +85,13 @@ void LegacyMetricsClient::StartReport() {
 
 void LegacyMetricsClient::Report(
     std::vector<fuchsia::legacymetrics::Event> events) {
-  DCHECK(metrics_recorder_);
   DVLOG(1) << __func__ << " called.";
 
+  // The connection might have dropped while additional metrics were being
+  // collected.
+  if (!metrics_recorder_)
+    return;
+
   // Include histograms.
   for (auto& histogram : GetLegacyMetricsDeltas()) {
     fuchsia::legacymetrics::Event histogram_event;

fuchsia/base/legacymetrics_client_unittest.cc

@@ -148,6 +148,31 @@ TEST_F(LegacyMetricsClientTest, AllTypes) {
   EXPECT_EQ("bar", events[2].user_action_event().name());
 }
 
+TEST_F(LegacyMetricsClientTest, DisconnectWhileCollectingAdditionalEvents) {
+  // Hold the completion callback for later execution.
+  base::OnceCallback<void(std::vector<fuchsia::legacymetrics::Event>)>
+      on_report_done;
+  client_.SetReportAdditionalMetricsCallback(base::BindRepeating(
+      [](base::OnceCallback<void(std::vector<fuchsia::legacymetrics::Event>)>*
+             stored_on_report_done,
+         base::OnceCallback<void(std::vector<fuchsia::legacymetrics::Event>)>
+             on_report_done) {
+        *stored_on_report_done = std::move(on_report_done);
+      },
+      base::Unretained(&on_report_done)));
+
+  client_.Start(kReportInterval);
+
+  task_environment_.FastForwardBy(kReportInterval);
+
+  // Disconnect the service.
+  service_binding_.reset();
+  base::RunLoop().RunUntilIdle();
+
+  // Fulfill the report additional metrics callback.
+  std::move(on_report_done).Run({});
+}
+
 TEST_F(LegacyMetricsClientTest, ReportSkippedNoEvents) {
   client_.Start(kReportInterval);