Kernel: Correctly set the lost_samples field for the first sample

This ensures that the lost_samples field is set to zero for the
first sample. We didn't lose any samples before the first sample
so this is the correct value. Without this Profiler gets confused
and draws the graph for the process which contains the first CPU
sample incorrectly (all zeroes usually).

Kernel/PerformanceEventBuffer.cpp

@@ -133,6 +133,7 @@ template<typename Serializer>
 bool PerformanceEventBuffer::to_json_impl(Serializer& object) const
 {
     auto array = object.add_array("events");
+    bool seen_first_sample = false;
     for (size_t i = 0; i < m_count; ++i) {
         auto& event = at(i);
         auto event_object = array.add_object();
@@ -183,7 +184,9 @@ bool PerformanceEventBuffer::to_json_impl(Serializer& object) const
         event_object.add("pid", event.pid);
         event_object.add("tid", event.tid);
         event_object.add("timestamp", event.timestamp);
-        event_object.add("lost_samples", i != 0 ? event.lost_samples : 0);
+        event_object.add("lost_samples", seen_first_sample ? event.lost_samples : 0);
+        if (event.type == PERF_EVENT_SAMPLE)
+            seen_first_sample = true;
         auto stack_array = event_object.add_array("stack");
         for (size_t j = 0; j < event.stack_size; ++j) {
             stack_array.add(event.stack[j]);