Add a strongly typed trace event for UMA histogram triggers in tracing

R=ssid@chromium.org

Change-Id: I2a981762ce1d46300488dfdbcfcb1ba87420b557
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1999324Reviewed-by: ssid <ssid@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731607}

content/browser/tracing/background_tracing_rule.cc

@@ -21,6 +21,8 @@
 #include "content/browser/tracing/background_tracing_manager_impl.h"
 #include "content/public/browser/browser_task_traits.h"
 #include "content/public/browser/browser_thread.h"
+#include "services/tracing/public/cpp/perfetto/macros.h"
+#include "third_party/perfetto/protos/perfetto/trace/track_event/chrome_histogram_sample.pbzero.h"
 
 namespace {
 
@@ -325,6 +327,15 @@ class HistogramRule : public BackgroundTracingRule,
                          TRACE_EVENT_SCOPE_THREAD, "histogram_name",
                          histogram_name, "value", actual_value);
 
+    TRACE_EVENT(
+        TRACE_DISABLED_BY_DEFAULT("histogram_samples"),
+        "HistogramSampleTrigger", [&](perfetto::EventContext ctx) {
+          perfetto::protos::pbzero::ChromeHistogramSample* new_sample =
+              ctx.event()->set_chrome_histogram_sample();
+          new_sample->set_name_hash(base::HashMetricName(histogram_name));
+          new_sample->set_sample(actual_value);
+        });
+
     OnHistogramTrigger(histogram_name);
   }