Stop using some v8::Context slot to find runtime.

Instead, use the object binding's pointer.  Puts the cart back
behind the horse.

Change-Id: I4c06ae991b871c6e90b0e6c70b69886addca2354
Reviewed-on: https://pdfium-review.googlesource.com/33630
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/fxjs/cjs_field.cpp b/fxjs/cjs_field.cpp
index 0a32247..cdbe0df 100644
--- a/fxjs/cjs_field.cpp
+++ b/fxjs/cjs_field.cpp
@@ -2279,10 +2279,8 @@
   if (pObj.IsEmpty())
     return CJS_Return(false);
 
-  CJS_Icon* pJS_Icon = static_cast<CJS_Icon*>(pRuntime->GetObjectPrivate(pObj));
-  if (!pJS_Icon)
-    return CJS_Return(false);
-  return CJS_Return(pJS_Icon->ToV8Object());
+  auto* pJS_Icon = static_cast<CJS_Icon*>(CFXJS_Engine::GetObjectPrivate(pObj));
+  return pJS_Icon ? CJS_Return(pJS_Icon->ToV8Object()) : CJS_Return(false);
 }
 
 CJS_Return CJS_Field::buttonImportIcon(
@@ -2399,8 +2397,8 @@
     if (pObj.IsEmpty())
       return CJS_Return(false);
 
-    CJS_Field* pJSField =
-        static_cast<CJS_Field*>(pRuntime->GetObjectPrivate(pObj));
+    auto* pJSField =
+        static_cast<CJS_Field*>(CFXJS_Engine::GetObjectPrivate(pObj));
     pJSField->AttachField(m_pJSDoc, *pStr);
     pRuntime->PutArrayElement(FormFieldArray, j++,
                               pJSField