Don't call SetProperty on the plugin unless HasProperty is true

The default implementation of SetProperty in the plugin (inside
ScriptableObject) will throw an exception. This can result in an
unexpected exception in JS when properties are set on the plugin
element. We should only call SetProperty if the plugin has that
property.

BUG=414842
TBR=dmichael@chromium.org

Review URL: https://codereview.chromium.org/640033003

Cr-Commit-Position: refs/heads/master@{#299040}

content/renderer/pepper/plugin_object.cc

@@ -97,6 +97,16 @@ bool PluginObject::SetNamedProperty(v8::Isolate* isolate,
                              StringVar::StringToPPVar(identifier));
   PepperTryCatchV8 try_catch(instance_, V8VarConverter::kAllowObjectVars,
                              isolate);
+
+  bool has_property =
+      ppp_class_->HasProperty(ppp_class_data_, identifier_var.get(),
+                              try_catch.exception());
+  if (try_catch.ThrowException())
+    return false;
+
+  if (!has_property)
+    return false;
+
   ScopedPPVar var = try_catch.FromV8(value);
   if (try_catch.ThrowException())
     return false;