[tracing] Don't register tracing-related allocations.

base/trace_event/heap_profiler_allocation_context_tracker.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_HEAP_PROFILER_ALLOCATION_CONTEXT_TRACKER_H_
 #define BASE_TRACE_EVENT_HEAP_PROFILER_ALLOCATION_CONTEXT_TRACKER_H_
 
 #include <vector>
 
 #include "base/atomicops.h"
@@ skipping 52 matching lines @@
   // Returns the thread-local instance, creating one if necessary. Returns
   // always a valid instance, unless it is called re-entrantly, in which case
   // returns nullptr in the nested calls.
   static AllocationContextTracker* GetInstanceForCurrentThread();
 
   // Set the thread name in the AllocationContextTracker of the current thread
   // if capture is enabled.
   static void SetCurrentThreadName(const char* name);
 
   // Starts and ends a new ignore scope between which the allocations are
-  // ignored in the heap profiler. A dummy context that short circuits to
-  // "tracing_overhead" is returned for these allocations.
+  // ignored by the heap profiler. GetContextSnapshot() returns false when
+  // allocations are ignored.
   void begin_ignore_scope() { ignore_scope_depth_++; }
   void end_ignore_scope() {
     if (ignore_scope_depth_)
       ignore_scope_depth_--;
   }
 
   // Pushes a frame onto the thread-local pseudo stack.
   void PushPseudoStackFrame(PseudoStackFrame stack_frame);
 
   // Pops a frame from the thread-local pseudo stack.
   void PopPseudoStackFrame(PseudoStackFrame stack_frame);
 
   // Push and pop current task's context. A stack is used to support nested
   // tasks and the top of the stack will be used in allocation context.
   void PushCurrentTaskContext(const char* context);
   void PopCurrentTaskContext(const char* context);
 
-  // Returns a snapshot of the current thread-local context.
-  AllocationContext GetContextSnapshot();
+  // Fills a snapshot of the current thread-local context. Doesn't fill and
+  // returns false if allocations are being ignored.
+  bool GetContextSnapshot(AllocationContext* snapshot);
 
   ~AllocationContextTracker();
 
  private:
   AllocationContextTracker();
 
   static subtle::Atomic32 capture_mode_;
 
   // The pseudo stack where frames are |TRACE_EVENT| names.
   std::vector<PseudoStackFrame> pseudo_stack_;
 
   // The thread name is used as the first entry in the pseudo stack.
   const char* thread_name_;
 
   // Stack of tasks' contexts. Context serves as a different dimension than
   // pseudo stack to cluster allocations.
   std::vector<const char*> task_contexts_;
 
   uint32_t ignore_scope_depth_;
 
   DISALLOW_COPY_AND_ASSIGN(AllocationContextTracker);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_HEAP_PROFILER_ALLOCATION_CONTEXT_TRACKER_H_