Re-land: base: Use MADV_REMOVE instead of ftruncate to purge discardable memory segments.

base/memory/discardable_shared_memory.h

Index: base/memory/discardable_shared_memory.h
diff --git a/base/memory/discardable_shared_memory.h b/base/memory/discardable_shared_memory.h
index 5dca884ca82e2d6665930905c92e990b8a5515c2..562295ce7e8c0e798317de4e8f61fa12ae576ef4 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -15,12 +15,6 @@
 #include <set>
 #endif
 
-// Define DISCARDABLE_SHARED_MEMORY_SHRINKING if platform supports shrinking
-// of discardable shared memory segments.
-#if defined(OS_POSIX) && !defined(OS_ANDROID)
-#define DISCARDABLE_SHARED_MEMORY_SHRINKING
-#endif
-
 namespace base {
 
 // Platform abstraction for discardable shared memory.
@@ -124,12 +118,6 @@ class BASE_EXPORT DiscardableSharedMemory {
     return shared_memory_.ShareToProcess(process_handle, new_handle);
   }
 
-#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
-  // Release as much memory as possible to the OS. The change in size will
-  // be reflected by the return value of mapped_size().
-  void Shrink();
-#endif
-
  private:
   // Virtual for tests.
   virtual Time Now() const;