Virtualize Observable<T>::ObservedPtr::OnDestroy() for CPDF_Avail cleanup

This enables more complicated cleanup when an observed object
is destroyed. Use it to make documents observable and to allow
the CPDF_Avail to cleanup without the need for intermediate class.

Change-Id: I3a8e758b7ff542e0a58710eff1ac8017205cbd45
Reviewed-on: https://pdfium-review.googlesource.com/36373
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/fxjs/cjs_runtime.cpp b/fxjs/cjs_runtime.cpp
index d7ee93f..f867e31 100644
--- a/fxjs/cjs_runtime.cpp
+++ b/fxjs/cjs_runtime.cpp
@@ -79,7 +79,7 @@
 }
 
 CJS_Runtime::~CJS_Runtime() {
-  NotifyObservedPtrs();
+  NotifyObservers();
   ReleaseEngine();
   if (m_isolateManaged) {
     GetIsolate()->Dispose();