Used a single class for SharedMemoryHandle.

base/memory/shared_memory_handle_posix.cc

Index: base/memory/shared_memory_handle_posix.cc
diff --git a/base/memory/shared_memory_handle_posix.cc b/base/memory/shared_memory_handle_posix.cc
index f11b35b1f8218437635047dba8a01e0dd9cca24e..3e9a403b14c23f8700cbf4105632c661edd6c6dc 100644
--- a/base/memory/shared_memory_handle_posix.cc
+++ b/base/memory/shared_memory_handle_posix.cc
@@ -12,6 +12,10 @@
 namespace base {
 
 SharedMemoryHandle::SharedMemoryHandle() = default;
+SharedMemoryHandle::SharedMemoryHandle(const SharedMemoryHandle& handle) =
+    default;
+SharedMemoryHandle& SharedMemoryHandle::operator=(
+    const SharedMemoryHandle& handle) = default;
 
 SharedMemoryHandle::SharedMemoryHandle(
     const base::FileDescriptor& file_descriptor)