Revert of base: Use MADV_REMOVE instead of ftruncate to purge discardable memory segments.

content/common/host_discardable_shared_memory_manager.cc

Index: content/common/host_discardable_shared_memory_manager.cc
diff --git a/content/common/host_discardable_shared_memory_manager.cc b/content/common/host_discardable_shared_memory_manager.cc
index e371c8c8795ad22aa2ce93754de6c98e04e2ebe5..90eee02594c38a9629b84a443c3ed0076ba2c9e2 100644
--- a/content/common/host_discardable_shared_memory_manager.cc
+++ b/content/common/host_discardable_shared_memory_manager.cc
@@ -325,9 +325,6 @@
     return;
   }
 
-  // Close file descriptor to avoid running out.
-  memory->Close();
-
   base::CheckedNumeric<size_t> checked_bytes_allocated = bytes_allocated_;
   checked_bytes_allocated += memory->mapped_size();
   if (!checked_bytes_allocated.IsValid()) {
@@ -337,6 +334,11 @@
 
   bytes_allocated_ = checked_bytes_allocated.ValueOrDie();
   BytesAllocatedChanged(bytes_allocated_);
+
+#if !defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
+  // Close file descriptor to avoid running out.
+  memory->Close();
+#endif
 
   scoped_refptr<MemorySegment> segment(new MemorySegment(memory.Pass()));
   process_segments[id] = segment.get();
@@ -429,6 +431,15 @@
 
     // Attempt to purge LRU segment. When successful, released the memory.
     if (segment->memory()->Purge(current_time)) {
+#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
+      size_t size = segment->memory()->mapped_size();
+      DCHECK_GE(bytes_allocated_, size);
+      bytes_allocated_ -= size;
+      // Shrink memory segment. This will immediately release the memory to
+      // the OS.
+      segment->memory()->Shrink();
+      DCHECK_EQ(segment->memory()->mapped_size(), 0u);
+#endif
       ReleaseMemory(segment->memory());
       continue;
     }