Avoid a CHECK failure inside CXFA_AttachNodeList::Insert().

Make sure the node to insert before, when it exists, is actually a
member of the CXFA_AttachNodeList. Give CXFA_List::Insert() a return
value, so when the validation fails, the caller can indicate failure.

BUG=pdfium:1263

Change-Id: Ia11f94a2407281694080d42cddbcc1c076bde5c9
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/52177
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fxjs/BUILD.gn b/fxjs/BUILD.gn
index ae3befc..30648f9 100644
--- a/fxjs/BUILD.gn
+++ b/fxjs/BUILD.gn
@@ -252,6 +252,7 @@
         "xfa/cfxjse_formcalc_context_embeddertest.cpp",
         "xfa/cfxjse_value_embeddertest.cpp",
         "xfa/cjx_hostpseudomodel_embeddertest.cpp",
+        "xfa/cjx_list_embeddertest.cpp",
       ]
       deps += [ "../xfa/fxfa" ]
     }