Add discardable memory emulation for non-android/mac platforms

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 d6e91b979d5a46fdd50e6ef07ab4f52a904bc042..70294fbb64d11c20db33d8c9a3a1d0206abe8dd9 100644
--- a/base/memory/discardable_memory.h
+++ b/base/memory/discardable_memory.h
@@ -17,6 +17,10 @@ enum LockDiscardableMemoryStatus {
   DISCARDABLE_MEMORY_SUCCESS = 1
 };
 
+#if defined(OS_MACOSX) || defined(OS_ANDROID)
+#define DISCARDABLE_MEMORY_SUPPORTED_NATIVELY
+#endif
+
 // Platform abstraction for discardable memory. DiscardableMemory is used to
 // cache large objects without worrying about blowing out memory, both on mobile
 // devices where there is no swap, and desktop devices where unused free memory
@@ -72,6 +76,8 @@ class BASE_EXPORT DiscardableMemory {
   // after every successful lock call.
   void Unlock();
 
+  bool is_locked() const { return is_locked_; }
+
   // Return the memory address held by this object. The object must be locked
   // before calling this. Otherwise, this will cause a DCHECK error.
   void* Memory() const;
@@ -87,6 +93,15 @@ class BASE_EXPORT DiscardableMemory {
   static void PurgeForTesting();
 
  private:
+#if !defined(DISCARDABLE_MEMORY_SUPPORTED_NATIVELY)

[Avi (use Gerrit), 2013/06/18 00:00:46]

How will this work on a platform like Win32 where we know if discardable memory
is available only at runtime? Will this turn into
DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY?

+  friend class DiscardableMemoryProvider;
+  friend class DiscardableMemoryProviderTest;
+
+  // Used by the DiscardableMemoryProvider to allocate and free memory.
+  bool Allocate();
+  void Deallocate();
+#endif
+
 #if defined(OS_ANDROID)
   // Maps the discardable memory into the caller's address space.
   // Returns true on success, false otherwise.