[Tracing] Add hook to PartitionAlloc for heap profiling

base/trace_event/memory_profiler_allocation_context.h

 // 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.
 
 #ifndef BASE_TRACE_EVENT_MEMORY_PROFILER_ALLOCATION_CONTEXT_H_
 #define BASE_TRACE_EVENT_MEMORY_PROFILER_ALLOCATION_CONTEXT_H_
 
 #include <vector>
 
 #include "base/atomicops.h"
@@ skipping 68 matching lines @@
     // higher than what can be stored here, the bottom frames are stored. Based
     // on the data above, a depth of 12 captures the full stack in the vast
     // majority of the cases.
     StackFrame frames[12];
   } backtrace;
 
   // There is room for two arbitrary context fields, which can be set by the
   // |TRACE_ALLOCATION_CONTEXT| macro. A nullptr key indicates that the field is
   // unused.
   std::pair<const char*, const char*> fields[2];
+
+  // Returns an |AllocationContext| with empty backtrace and no fields.
+  static AllocationContext GetEmpty();
 };
 
 // The allocation context tracker keeps track of thread-local context for heap
 // profiling. It includes a pseudo stack of trace events, and it might contain
 // arbitrary (key, value) context. On every allocation the tracker provides a
 // snapshot of its context in the form of an |AllocationContext| that is to be
 // stored together with the allocation details.
 class BASE_EXPORT AllocationContextTracker {
  public:
   // Globally enables capturing allocation context.
@@ skipping 47 matching lines @@
   // A dictionary of arbitrary context.
   SmallMap<std::map<const char*, const char*>> context_;
 
   DISALLOW_COPY_AND_ASSIGN(AllocationContextTracker);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_MEMORY_PROFILER_ALLOCATION_CONTEXT_H_