Add DiscardableMemoryAllocator to work around FD limit issue.

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 c6a8d82da72363d3dc2cecbb01547eeb64c226eb..e060dd0accc89c4cb6ad0627410dc9f7f0119459 100644
--- a/base/memory/discardable_memory.h
+++ b/base/memory/discardable_memory.h
@@ -30,6 +30,9 @@ enum LockDiscardableMemoryStatus {
 // reclaim it if needed. Locks do not nest.
 //
 // Notes:
+//   - If you need more than one instance of DiscardableMemory please use
+//     DiscardableMemoryAllocator to avoid using too many file descriptors on
+//     platforms where discardable memory is backed by a file.
 //   - The paging behavior of memory while it is locked is not specified. While
 //     mobile platforms will not swap it out, it may qualify for swapping
 //     on desktop platforms. It is not expected that this will matter, as the
@@ -53,6 +56,8 @@ class BASE_EXPORT DiscardableMemory {
 
   static scoped_ptr<DiscardableMemory> CreateLockedMemory(size_t size);
 
+  size_t size() const { return size_; }
+
   // Locks the memory so that it will not be purged by the system. Returns
   // DISCARDABLE_MEMORY_SUCCESS on success. If the return value is
   // DISCARDABLE_MEMORY_FAILED then this object should be discarded and
@@ -78,6 +83,11 @@ class BASE_EXPORT DiscardableMemory {
   // Purge all discardable memory in the system. This call has global effects
   // across all running processes, so it should only be used for testing!
   static void PurgeForTesting();
+
+ protected:
+  DiscardableMemory(size_t size);
+
+  const size_t size_;
 };
 
 }  // namespace base