Do IWYU for unowned_ptr.h, and remove some unused includes.

git grep -l UnownedPtr|grep h$|xargs grep -L unowned_ptr.h|xargs echo

Change-Id: Id34f35355a893c596d0fc865db33a9ba9f684cae
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/74031
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fxge/cfx_fontmapper.h b/core/fxge/cfx_fontmapper.h
index 67be9da..51f5d16 100644
--- a/core/fxge/cfx_fontmapper.h
+++ b/core/fxge/cfx_fontmapper.h
@@ -13,6 +13,7 @@
 
 #include "core/fxcrt/fx_memory_wrappers.h"
 #include "core/fxcrt/fx_string.h"
+#include "core/fxcrt/unowned_ptr.h"
 #include "core/fxge/cfx_face.h"
 #include "third_party/base/optional.h"