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

base/memory/discardable_memory.h

Index: base/memory/discardable_memory.h
diff --git a/base/memory/discardable_memory.h b/base/memory/discardable_memory.h
index fc189e746341136d0190e0329d3c375be8500af2..e5ba7cc5602011ffce68ba43dc9ce18ab3891c3b 100644
--- a/base/memory/discardable_memory.h
+++ b/base/memory/discardable_memory.h
@@ -10,6 +10,10 @@
 #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
@@ -59,6 +63,14 @@ class BASE_EXPORT DiscardableMemory {
   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