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..0cae99f5494d1ad1cd7a8d3600982ee772dc0771 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
@@ -44,6 +47,8 @@ enum LockDiscardableMemoryStatus {
 //   - Mac: http://trac.webkit.org/browser/trunk/Source/WebCore/platform/mac/PurgeableBufferMac.cpp
 //          the comment starting with "vm_object_purgable_control" at
 //            http://www.opensource.apple.com/source/xnu/xnu-792.13.8/osfmk/vm/vm_object.c
+//
+// Thread-safety: DiscardableMemory instances are not thread-safe.
 class BASE_EXPORT DiscardableMemory {
  public:
   virtual ~DiscardableMemory() {}
@@ -53,6 +58,10 @@ class BASE_EXPORT DiscardableMemory {
 
   static scoped_ptr<DiscardableMemory> CreateLockedMemory(size_t size);
 
+  // Returns the memory's actual size that is greater or equal than the
+  // size requested at construction.
+  virtual size_t Size() const = 0;
+
   // 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