Slow Reports - Add message filter to trigger background traces from child processes.

components/tracing/child_trace_message_filter.h

Index: components/tracing/child_trace_message_filter.h
diff --git a/components/tracing/child_trace_message_filter.h b/components/tracing/child_trace_message_filter.h
index f61e29429e2f0221a5edfc59042bfb7c54c2692a..04c45c7a86a030b944451ff9cab41c3e794cc1d6 100644
--- a/components/tracing/child_trace_message_filter.h
+++ b/components/tracing/child_trace_message_filter.h
@@ -7,6 +7,7 @@
 
 #include "base/bind.h"
 #include "base/memory/ref_counted_memory.h"
+#include "base/metrics/histogram.h"
 #include "base/trace_event/memory_dump_request_args.h"
 #include "ipc/message_filter.h"
 
@@ -57,6 +58,12 @@ class ChildTraceMessageFilter : public IPC::MessageFilter {
   void OnProcessMemoryDumpRequest(
       const base::trace_event::MemoryDumpRequestArgs& args);
   void OnGlobalMemoryDumpResponse(uint64 dump_guid, bool success);
+  void OnSetUMACallback(const std::string& histogram_name, int histogram_value);
+  void OnClearUMACallback(const std::string& histogram_name);
+  void OnHistogramChanged(const std::string& histogram_name,
+                          base::Histogram::Sample reference_value,
+                          base::Histogram::Sample actual_value);
+  void SendTriggerMessage(const std::string& histogram_name);
 
   // Callback from trace subsystem.
   void OnTraceDataCollected(