base: Rename discardable memory allocator interface and remove unnecessary class.

base/memory/discardable_memory.cc

Index: base/memory/discardable_memory.cc
diff --git a/base/memory/discardable_memory.cc b/base/memory/discardable_memory.cc
index 0e3b58a356390ba604aea22d3f692701eba09fff..a99c6f8de4be11166c862546e261fec63dd18bbe 100644
--- a/base/memory/discardable_memory.cc
+++ b/base/memory/discardable_memory.cc
@@ -4,14 +4,15 @@
 
 #include "base/memory/discardable_memory.h"
 
-#include "base/memory/discardable_memory_shmem.h"
+#include "base/memory/discardable_memory_allocator.h"
 
 namespace base {
 
 // static
 scoped_ptr<DiscardableMemory> DiscardableMemory::CreateLockedMemory(
     size_t size) {
-  return make_scoped_ptr(new internal::DiscardableMemoryShmem(size));
+  return DiscardableMemoryAllocator::GetInstance()
+      ->AllocateLockedDiscardableMemory(size);
 }
 
 }  // namespace base