[Tracing] Add hook to PartitionAlloc for heap profiling

base/trace_event/memory_profiler_allocation_context.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/trace_event/memory_profiler_allocation_context.h"
 
 #include <algorithm>
 
+#include "base/macros.h"
 #include "base/threading/thread_local_storage.h"
 
 namespace base {
 namespace trace_event {
 
 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) {
   delete static_cast<AllocationContextTracker*>(alloc_ctx_tracker);
 }
 
 AllocationContextTracker* AllocationContextTracker::GetThreadLocalTracker() {
   auto tracker =
       static_cast<AllocationContextTracker*>(g_tls_alloc_ctx_tracker.Get());
 
@@ skipping 90 matching lines @@
     // If there is room for more, fill the remaining slots with nullptr keys.
     for (; dst != dst_end; dst++)
       dst->first = nullptr;
   }
 
   return ctx;
 }
 
 }  // namespace trace_event
 }  // namespace base