Simplify IsObjectStream() inside cpdf_object_stream.cpp

Change the IsObjectStream() signature to take a CPDF_Stream*, since that
is what all the callers pass in. Then IsObjectStream() does not need to
verify if the input is a CPDF_Stream.

Change-Id: I6039c3fa0dcf0263bc705e9967775fb2a6fefb20
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/115612
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fpdfapi/parser/cpdf_object_stream.cpp b/core/fpdfapi/parser/cpdf_object_stream.cpp
index 19294da..16dc92c 100644
--- a/core/fpdfapi/parser/cpdf_object_stream.cpp
+++ b/core/fpdfapi/parser/cpdf_object_stream.cpp
@@ -21,8 +21,7 @@
 
 namespace {
 
-bool IsObjectStream(const CPDF_Object* object) {
-  const CPDF_Stream* stream = ToStream(object);
+bool IsObjectStream(const CPDF_Stream* stream) {
   if (!stream)
     return false;