[Tracing] Add hook to PartitionAlloc for heap profiling

base/trace_event/memory_profiler_allocation_context.cc

Index: base/trace_event/memory_profiler_allocation_context.cc
diff --git a/base/trace_event/memory_profiler_allocation_context.cc b/base/trace_event/memory_profiler_allocation_context.cc
index b82e2109845b0494c82ee7d689aefe5f7a7d960b..ddd74640ea2c98638d2462ad3c1098469e7e8429 100644
--- a/base/trace_event/memory_profiler_allocation_context.cc
+++ b/base/trace_event/memory_profiler_allocation_context.cc
@@ -6,6 +6,7 @@
 
 #include <algorithm>
 
+#include "base/macros.h"
 #include "base/threading/thread_local_storage.h"
 
 namespace base {
@@ -15,11 +16,24 @@ subtle::Atomic32 AllocationContextTracker::capture_enabled_ = 0;
 
 namespace {
 ThreadLocalStorage::StaticSlot g_tls_alloc_ctx_tracker = TLS_INITIALIZER;
-}
+
+}  // namespace
 
 AllocationStack::AllocationStack() {}
 AllocationStack::~AllocationStack() {}
 
+AllocationContext AllocationContext::GetEmpty() {
+  AllocationContext context;
+
+  for (size_t i = 0; i < arraysize(context.backtrace.frames); i++)
+    context.backtrace.frames[i] = nullptr;
+
+  for (size_t i = 0; i < arraysize(context.fields); i++)
+    context.fields[i].first = nullptr;
+
+  return context;
+}
+
 // This function is added to the TLS slot to clean up the instance when the
 // thread exits.
 void DestructAllocationContextTracker(void* alloc_ctx_tracker) {