[tracing] Introduce support for userland memory allocators dumps.

base/trace_event/process_memory_totals.h

Index: base/trace_event/process_memory_totals.h
diff --git a/base/trace_event/process_memory_totals.h b/base/trace_event/process_memory_totals.h
index 1c99152fa84b6086163393c247be39d146ee0334..ef0289ee3d8c643b99d492a31e40dff84b0eb98d 100644
--- a/base/trace_event/process_memory_totals.h
+++ b/base/trace_event/process_memory_totals.h
@@ -16,7 +16,7 @@ class TracedValue;
 // Dump provider which collects process-wide memory stats.
 class BASE_EXPORT ProcessMemoryTotals {
  public:
-  ProcessMemoryTotals() {}
+  ProcessMemoryTotals() : resident_set_bytes_(0) {}
 
   // Called at trace generation time to populate the TracedValue.
   void AsValueInto(TracedValue* value) const;