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..c173dfe1cde5a2076b655aa52306d814e7196e4c 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

[Avi (use Gerrit), 2013/06/20 21:16:41]

If there is now a runtime call, do we need this #define?

[Ian Vollick, 2013/06/20 23:27:09]

Yup, but it needs a better name. We need to conditionally compile code if
there's any chance we need emulation. I've changed this to
DISCARDABLE_MEMORY_ALWAYS_SUPPORTED_NATIVELY. For the unit test, I've also added
DISCARDABLE_MEMORY_NEVER_SUPPORTED_NATIVELY. For the win implementation, neither
would be defined. 

[Avi (use Gerrit), 2013/06/20 23:29:56]

Meh. I'm not thrilled, but I don't care that much.

+#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
@@ -51,8 +55,9 @@ class BASE_EXPORT DiscardableMemory {
   // The opened file will also be closed after this.
   ~DiscardableMemory();
 
-  // Check whether the system supports discardable memory.
-  static bool Supported();
+  // Check whether the system supports discardable memory natively. Returns
+  // false if the support is emulated.
+  static bool SupportedNatively();
 
   // Initialize the DiscardableMemory object. On success, this function returns
   // true and the memory is locked. This should only be called once.
@@ -72,6 +77,8 @@ 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 +94,15 @@ class BASE_EXPORT DiscardableMemory {
   static void PurgeForTesting();
 
  private:
+#if !defined(DISCARDABLE_MEMORY_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.