[Tracing] Add hook to PartitionAlloc for heap profiling

base/trace_event/memory_profiler_allocation_context.h

Index: base/trace_event/memory_profiler_allocation_context.h
diff --git a/base/trace_event/memory_profiler_allocation_context.h b/base/trace_event/memory_profiler_allocation_context.h
index 3487f3ecfe0a9271ae768e03cc7892a64b7e68d2..41ab427ed168d1dfa9e8c7ac1e1fac46cca5d27a 100644
--- a/base/trace_event/memory_profiler_allocation_context.h
+++ b/base/trace_event/memory_profiler_allocation_context.h
@@ -86,6 +86,9 @@ struct BASE_EXPORT AllocationContext {
   // |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