Use DiscardableMemoryManager on Android.

base/memory/discardable_memory_manager.h

Index: base/memory/discardable_memory_manager.h
diff --git a/base/memory/discardable_memory_manager.h b/base/memory/discardable_memory_manager.h
index 844752151e64fe0b06df2a88fe2ab43da0fe619e..0a25622d9d0db4d40d00dfa9da58aca1f0886d2e 100644
--- a/base/memory/discardable_memory_manager.h
+++ b/base/memory/discardable_memory_manager.h
@@ -16,6 +16,11 @@ namespace internal {
 
 // This interface is used by the DiscardableMemoryManager class to provide some
 // level of userspace control over discardable memory allocations.
+//
+// Thread-safety: The methods below are called while the manager's mutex is
+// acquired. This means that subclasses don't need to deal with synchronization
+// as long as they correctly go through the manager's methods (e.g.
+// AcuireLock(), ReleaseLock()...).

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

It's really the usage by the manager rather than the interface itself that
provides this guarantee. How about we move this comment to the Manager below?
Maybe next to the Register() function. ie. "allocations registered with the
manager will never be used concurrently.."

 class DiscardableMemoryManagerAllocation {
  public:
   // Allocate and acquire a lock that prevents the allocation from being purged