Add WTF::DiscardableMemory.

Source/wtf/DiscardableMemory.h

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-#ifndef WebDiscardableMemory_h
-#define WebDiscardableMemory_h
+#ifndef WTF_DiscardableMemory_h
+#define WTF_DiscardableMemory_h
 
-namespace blink {
+namespace WTF {
 
 // A memory allocation that can be automatically discarded by the operating
 // system under memory pressure.
 //
 // Discardable usage is typically:
-//    WebDiscardableMemory* mem = allocateAndLockedDiscardableMemory(1024*1024);
+//    DiscardableMemory* mem = WTFPlatform::getInstance()->allocateAndLockedDiscardableMemory(1024*1024);
 //    void* data = mem->data();
 //    memset(data, 3, 1024*1024);
 //    mem->unlock();
 //
 // Later, when you need it again:
 //    if (!mem->lock()) {
 //       ... handle the fact that the memory is gone...
 //       delete mem; // Make sure to destroy it. It is never going to come back.
 //       return;
 //    }
 //    ... use mem->data() as much as you want
 //    mem->unlock();
 //
-class WebDiscardableMemory {
+class DiscardableMemory {
 public:
     // Must not be called while locked.
-    virtual ~WebDiscardableMemory() { }
+    virtual ~DiscardableMemory() { }
 
     // Locks the memory, prevent it from being discarded. Once locked. you may
     // obtain a pointer to that memory using the data() method.
     //
     // lock() may return false, indicating that the underlying memory was
-    // discarded and that the lock failed. In this case, the
-    // WebDiscardableMemory is effectively dead.
+    // discarded and that the lock failed. In this case, the DiscardableMemory
+    // is effectively dead.
     //
     // Nested calls to lock are not allowed.
     virtual bool lock() = 0;
 
     // Returns the current pointer for the discardable memory. This call is ONLY
     // valid when the discardable memory object is locked.
     virtual void* data() = 0;
 
     // Unlock the memory so that it can be purged by the system. Must be called
     // after every successful lock call.
     virtual void unlock() = 0;
 };
 
-} // namespace blink
+} // namespace WTF
 
 #endif