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

content/common/discardable_shared_memory_heap.cc

 // Copyright 2014 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.
 
 #include "content/common/discardable_shared_memory_heap.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/format_macros.h"
@@ skipping 400 matching lines @@
                       base::trace_event::MemoryAllocatorDump::kUnitsBytes,
                       allocated_objects_size_in_bytes);
   obj_dump->AddScalar("locked_size",
                       base::trace_event::MemoryAllocatorDump::kUnitsBytes,
                       locked_objects_size_in_bytes);
 
   // Emit an ownership edge towards a global allocator dump node. This allows
   // to avoid double-counting segments when both browser and child process emit
   // them. In the special case of single-process-mode, this will be the only
   // dumper active and the single ownership edge will become a no-op in the UI.
+  // The global dump is created as a weak dump so that the segment is removed if
+  // the browser does not dump it (segment was purged).
   const uint64_t tracing_process_id =
       base::trace_event::MemoryDumpManager::GetInstance()
           ->GetTracingProcessId();
   base::trace_event::MemoryAllocatorDumpGuid shared_segment_guid =
       GetSegmentGUIDForTracing(tracing_process_id, segment_id);
-  pmd->CreateSharedGlobalAllocatorDump(shared_segment_guid);
+  pmd->CreateWeakSharedGlobalAllocatorDump(shared_segment_guid);
 
   // The size is added to the global dump so that it gets propagated to both the
   // dumps associated.
   pmd->GetSharedGlobalAllocatorDump(shared_segment_guid)
       ->AddScalar(base::trace_event::MemoryAllocatorDump::kNameSize,
                   base::trace_event::MemoryAllocatorDump::kUnitsBytes,
                   allocated_objects_size_in_bytes);
 
   // By creating an edge with a higher |importance| (w.r.t. browser-side dumps)
   // the tracing UI will account the effective size of the segment to the child.
@@ skipping 26 matching lines @@
   ScopedVector<ScopedMemorySegment>::const_iterator it =
       std::find_if(memory_segments_.begin(), memory_segments_.end(),
                    [span](const ScopedMemorySegment* segment) {
                      return segment->ContainsSpan(span);
                    });
   DCHECK(it != memory_segments_.end());
   return (*it)->CreateMemoryAllocatorDump(span, block_size_, name, pmd);
 }
 
 }  // namespace content