Make CPDFXFA_WidgetHandler::NewAnnotForXFA() return a std::unique_ptr.

Fix its call stack to better indicate ownership, and remove impossible
checks as well.

Change-Id: If46cc6ef091139c32f8eaf00bb6f213b16b9b0fb
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/61671
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_annothandlermgr.cpp b/fpdfsdk/cpdfsdk_annothandlermgr.cpp
index b077532..2a45b0d 100644
--- a/fpdfsdk/cpdfsdk_annothandlermgr.cpp
+++ b/fpdfsdk/cpdfsdk_annothandlermgr.cpp
@@ -58,7 +58,7 @@
 }
 
 #ifdef PDF_ENABLE_XFA
-CPDFSDK_Annot* CPDFSDK_AnnotHandlerMgr::NewXFAAnnot(
+std::unique_ptr<CPDFSDK_Annot> CPDFSDK_AnnotHandlerMgr::NewXFAAnnot(
     CXFA_FFWidget* pAnnot,
     CPDFSDK_PageView* pPageView) {
   ASSERT(pAnnot);