[tracing] Simplify design of MemoryAllocatorDump

base/trace_event/process_memory_dump.h

Index: base/trace_event/process_memory_dump.h
diff --git a/base/trace_event/process_memory_dump.h b/base/trace_event/process_memory_dump.h
index 3d66f78e56dabff61480102060a0e8f1cb4c9d36..bd9c5434ddabfc624b0a804c1c5744104a180f5d 100644
--- a/base/trace_event/process_memory_dump.h
+++ b/base/trace_event/process_memory_dump.h
@@ -30,6 +30,8 @@ class MemoryDumpSessionState;
 // dump point time.
 class BASE_EXPORT ProcessMemoryDump {
  public:
+  // Maps allocator dumps absolute names (allocator_name/heap/subheap) to
+  // MemoryAllocatorDump instances.
   using AllocatorDumpsMap =
       SmallMap<hash_map<std::string, MemoryAllocatorDump*>>;
 
@@ -48,16 +50,26 @@ class BASE_EXPORT ProcessMemoryDump {
   void set_has_process_mmaps() { has_process_mmaps_ = true; }
 
   // Creates a new MemoryAllocatorDump with the given name and returns the
-  // empty object back to the caller. The |name| must be unique in the dump.
-  // ProcessMemoryDump handles the memory ownership of the created object.
-  // |parent| can be used to specify a hierarchical relationship of the
-  // allocator dumps.
-  MemoryAllocatorDump* CreateAllocatorDump(const std::string& name);
-  MemoryAllocatorDump* CreateAllocatorDump(const std::string& name,
-                                           MemoryAllocatorDump* parent);
-
-  // Returns a MemoryAllocatorDump given its name or nullptr if not found.
-  MemoryAllocatorDump* GetAllocatorDump(const std::string& name) const;
+  // empty object back to the caller.
+  // Arguments:
+  //   allocator_name: a name that univocally identifies allocator dumps
+  //     produced by this provider. It acts as a type w.r.t. the allocator
+  //     attributes, in the sense that all the MAD with the same allocator_name
+  //     are expected to have the same attributes.
+  //   heap_name, either:
+  //     - kRootHeap: if the allocator has only one default heap.
+  //     - a string identifing a heap name (e.g., isolate1, isolate2 ...). It is
+  //       possible to specify nesting by using a path-like string (e.g.,
+  //       isolate1/heap_spaceX, isolate1/heap_spaceY, isolate2/heap_spaceX).
+  // The tuple (|allocator_name|, |heap_name|) is unique inside a PMD.
+  // ProcessMemoryDump handles the memory ownership of its MemoryAllocatorDumps.
+  MemoryAllocatorDump* CreateAllocatorDump(const std::string& allocator_name,
+                                           const std::string& heap_name);
+
+  // Looks up a MemoryAllocatorDump given its allocator and heap names, or
+  // nullptr if not found.
+  MemoryAllocatorDump* GetAllocatorDump(const std::string& allocator_name,
+                                        const std::string& heap_name) const;
 
   // Returns the map of the MemoryAllocatorDumps added to this dump.
   const AllocatorDumpsMap& allocator_dumps() const { return allocator_dumps_; }