Observe CXFA_FFWidgets in CPDFSDK_XFAWidget.

CXFA_FFWidgets, which bridge the layout tree and the parser tree,
are ephemeral, and go out of existence under certain events.

- Tidy member name.

Bug: chromium:982828
Change-Id: Iccc9b6f75dcd6a8fd2af98d40b0f1c13ffa51816
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/57572
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

fpdfsdk/cpdfsdk_xfawidget.cpp

diff --git a/fpdfsdk/cpdfsdk_xfawidget.cpp b/fpdfsdk/cpdfsdk_xfawidget.cpp
index a424d26..f529a72 100644
--- a/fpdfsdk/cpdfsdk_xfawidget.cpp
+++ b/fpdfsdk/cpdfsdk_xfawidget.cpp
@@ -14,7 +14,7 @@
                                      CPDFSDK_InteractiveForm* pInteractiveForm)
     : CPDFSDK_Annot(pPageView),
       m_pInteractiveForm(pInteractiveForm),
-      m_hXFAWidget(pAnnot) {}
+      m_pXFAWidget(pAnnot) {}
 
 CPDFSDK_XFAWidget::~CPDFSDK_XFAWidget() = default;
 
@@ -23,7 +23,7 @@
 }
 
 CXFA_FFWidget* CPDFSDK_XFAWidget::GetXFAWidget() const {
-  return m_hXFAWidget.Get();
+  return m_pXFAWidget.Get();
 }
 
 CPDF_Annot::Subtype CPDFSDK_XFAWidget::GetAnnotSubtype() const {

fpdfsdk/cpdfsdk_xfawidget.h

diff --git a/fpdfsdk/cpdfsdk_xfawidget.h b/fpdfsdk/cpdfsdk_xfawidget.h
index 078f011..3ee5cde 100644
--- a/fpdfsdk/cpdfsdk_xfawidget.h
+++ b/fpdfsdk/cpdfsdk_xfawidget.h
@@ -9,6 +9,7 @@
 
 #include "core/fxcrt/fx_coordinates.h"
 #include "core/fxcrt/fx_string.h"
+#include "core/fxcrt/observed_ptr.h"
 #include "core/fxcrt/unowned_ptr.h"
 #include "fpdfsdk/cpdfsdk_annot.h"
 
@@ -35,7 +36,7 @@
 
  private:
   UnownedPtr<CPDFSDK_InteractiveForm> const m_pInteractiveForm;
-  UnownedPtr<CXFA_FFWidget> const m_hXFAWidget;
+  ObservedPtr<CXFA_FFWidget> const m_pXFAWidget;
 };
 
 inline CPDFSDK_XFAWidget* ToXFAWidget(CPDFSDK_Annot* pAnnot) {