Do IWYU for retain_ptr.h in headers.

Headers that reference RetainPtr or CONSTRUCT_VIA_MAKE_RETAIN should
include retain_ptr.h.

Change-Id: If1baf1036621032a7e7eaa57e1c31522a5813114
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/79299
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/testing/invalid_seekable_read_stream.h b/testing/invalid_seekable_read_stream.h
index 8eee02d..85c2084 100644
--- a/testing/invalid_seekable_read_stream.h
+++ b/testing/invalid_seekable_read_stream.h
@@ -6,6 +6,7 @@
 #define TESTING_INVALID_SEEKABLE_READ_STREAM_H_
 
 #include "core/fxcrt/fx_stream.h"
+#include "core/fxcrt/retain_ptr.h"
 
 // A stream used for testing where reads always fail.
 class InvalidSeekableReadStream final : public IFX_SeekableReadStream {