Convert AutoClosedCommand::stream_ to UnownedPtr<>

Mark as const while at it.

Change-Id: I49a73eb9236ee78e656004341fa829829ad4e527
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/89210
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_appstream.cpp b/fpdfsdk/cpdfsdk_appstream.cpp
index 2dd0b27..3d68836 100644
--- a/fpdfsdk/cpdfsdk_appstream.cpp
+++ b/fpdfsdk/cpdfsdk_appstream.cpp
@@ -95,7 +95,7 @@
   virtual ~AutoClosedCommand() { *stream_ << close_ << "\n"; }
 
  private:
-  std::ostringstream* stream_;
+  UnownedPtr<std::ostringstream> const stream_;
   ByteString close_;
 };