Remove some ASSERT (and cast) in favor of checked cases.

Because it is a stronger pattern at runtime.
These were found by essentially:  grep -ni '\bassert\b.*type'

Change-Id: I913d77139053e8980528597a6633e1859e5204c4
Reviewed-on: https://pdfium-review.googlesource.com/38890
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_widget.h b/fpdfsdk/cpdfsdk_widget.h
index 97971c8..1349981 100644
--- a/fpdfsdk/cpdfsdk_widget.h
+++ b/fpdfsdk/cpdfsdk_widget.h
@@ -144,4 +144,10 @@
 #endif  // PDF_ENABLE_XFA
 };
 
+inline CPDFSDK_Widget* ToCPDFSDKWidget(CPDFSDK_Annot* pAnnot) {
+  return pAnnot && pAnnot->GetAnnotSubtype() == CPDF_Annot::Subtype::WIDGET
+             ? static_cast<CPDFSDK_Widget*>(pAnnot)
+             : nullptr;
+}
+
 #endif  // FPDFSDK_CPDFSDK_WIDGET_H_