Use DiscardableMemoryManager on Android.

base/memory/discardable_memory_manager.cc

Index: base/memory/discardable_memory_manager.cc
diff --git a/base/memory/discardable_memory_manager.cc b/base/memory/discardable_memory_manager.cc
index 1dff3e4da0ec396a8393de9d1607ad49dd04241c..31f9b451cef16b5516495eaab8ba7008626ea637 100644
--- a/base/memory/discardable_memory_manager.cc
+++ b/base/memory/discardable_memory_manager.cc
@@ -84,7 +84,16 @@ void DiscardableMemoryManager::Unregister(Allocation* allocation) {
     DCHECK_LE(bytes_purgable, bytes_allocated_);
     bytes_allocated_ -= bytes_purgable;
     BytesAllocatedChanged();
+  } else {
+    // Note that Purge() which is called below requires the Allocation instance
+    // to be unlocked.
+    allocation->ReleaseLock();

[reveman, 2014/04/26 00:13:37]

It's currently up to the client to call DiscardableMemoryManager::ReleaseLock
before Unregister. Accounting will be incorrect if Unregister is called for an
allocation which a lock is currently acquired.

   }
+
+  // Purge the allocation here while the mutex is acquired to let the underlying
+  // Allocation instance not have to deal with synchronization on its own.
+  allocation->Purge();

[reveman, 2014/04/26 00:13:37]

I don't think we should do this here. I prefer to have Unregister simply stop
tracking the allocation and leave it up to the client to decide what to do with
it.

Maybe add a DiscardableMemoryManager::Purge(Allocation* allocation) function
instead that you can call prior to Unregister()?

+
   allocations_.Erase(it);
 }