[tracing] Add method to create "weak" global dumps

base/trace_event/process_memory_dump.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_PROCESS_MEMORY_DUMP_H_
 #define BASE_TRACE_EVENT_PROCESS_MEMORY_DUMP_H_
 
 #include <stddef.h>
 
 #include <vector>
@@ skipping 80 matching lines @@
   MemoryAllocatorDump* GetOrCreateAllocatorDump(
       const std::string& absolute_name);
 
   // Creates a shared MemoryAllocatorDump, to express cross-process sharing.
   // Shared allocator dumps are allowed to have duplicate guids within the
   // global scope, in order to reference the same dump from multiple processes.
   // See the design doc goo.gl/keU6Bf for reference usage patterns.
   MemoryAllocatorDump* CreateSharedGlobalAllocatorDump(
       const MemoryAllocatorDumpGuid& guid);
 
+  // Creates a shared MemoryAllocatorDump as CreateSharedGlobalAllocatorDump,
+  // but with a WEAK flag. A weak dump will be removed unless a non-weak dump

[Primiano Tucci (use gerrit), 2016/01/19 17:07:11]

s/removed/discarded/ also below

[ssid, 2016/01/20 18:58:34]

Done.

+  // is created using CreateSharedGlobalAllocatorDump by at least one process
+  // (order of creation does not matter). All owners and children of the removed
+  // dump will also be removed transitively.
+  MemoryAllocatorDump* CreateWeakSharedGlobalAllocatorDump(

[Primiano Tucci (use gerrit), 2016/01/19 17:07:11]

You want to clarify that:
The Weak flag applies only if:
 - No other non-weak dump, with the same GUID, exists already. Otherwise the
dump stays non-weak.
 - No further non-weak dump with the same GUID is created. Otherwise the dump
will become non-weak.

[ssid, 2016/01/20 18:58:34]

Done.

+      const MemoryAllocatorDumpGuid& guid);
+
   // Looks up a shared MemoryAllocatorDump given its guid.
   MemoryAllocatorDump* GetSharedGlobalAllocatorDump(
       const MemoryAllocatorDumpGuid& guid) const;
 
   // Returns the map of the MemoryAllocatorDumps added to this dump.
   const AllocatorDumpsMap& allocator_dumps() const { return allocator_dumps_; }
 
   // Adds a heap dump for the allocator with |absolute_name|. The |TracedValue|
   // must have the correct format. |trace_event::HeapDumper| will generate such
   // a value from a |trace_event::AllocationRegister|.
@@ skipping 71 matching lines @@
   // Keeps track of relationships between MemoryAllocatorDump(s).
   std::vector<MemoryAllocatorDumpEdge> allocator_dumps_edges_;
 
   DISALLOW_COPY_AND_ASSIGN(ProcessMemoryDump);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_PROCESS_MEMORY_DUMP_H_