Add support to create memory allocator dump in base::DiscardableMemory

base/memory/discardable_memory.h

 // Copyright (c) 2013 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_MEMORY_DISCARDABLE_MEMORY_H_
 #define BASE_MEMORY_DISCARDABLE_MEMORY_H_
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 
 namespace base {
+namespace trace_event {
+class MemoryAllocatorDump;
+class ProcessMemoryDump;
+}
 
 // Discardable memory is used to cache large objects without worrying about
 // blowing out memory, both on mobile devices where there is no swap, and
 // desktop devices where unused free memory should be used to help the user
 // experience. This is preferable to releasing memory in response to an OOM
 // signal because it is simpler and provides system-wide management of
 // purgable memory, though it has less flexibility as to which objects get
 // discarded.
 //
 // Discardable memory has two states: locked and unlocked. While the memory is
@@ skipping 29 matching lines @@
   virtual void Unlock() = 0;
 
   // Returns the memory address held by this object. The object must be locked
   // before calling this.
   virtual void* data() const = 0;
 
   // Handy method to simplify calling data() with a reinterpret_cast.
   template<typename T> T* data_as() const {
     return reinterpret_cast<T*>(data());
   }
+
+  // Used for dumping the statistics of discardable memory allocated in tracing.
+  // Returns nullptr if the memory is purged by the allocator. Returns a new

[reveman, 2015/08/21 18:05:24]

"Returns nullptr if the memory is purged by the allocator" can you remove this
part and have it always return a dump for now.

[ssid, 2015/08/21 19:02:30]

Yes that makes sense, I will make it dump size 0.

[ssid, 2015/08/24 14:52:09]

Done.

+  // MemoryAllocatorDump in the |pmd| with the size of the discardable memory.
+  // See ProcessMemoryDump::CreateAllocatorDump.
+  virtual trace_event::MemoryAllocatorDump* CreateMemoryAllocatorDump(
+      const char* name,
+      trace_event::ProcessMemoryDump* pmd) = 0;
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_MEMORY_H_