Changes PepperPluginInstanceImpl::IsRectTopmost to not use

container_ if we are destroying the instance.

BUG=476590

Review URL: https://codereview.chromium.org/1085423004

Cr-Commit-Position: refs/heads/master@{#325561}

content/renderer/pepper/pepper_plugin_instance_impl.cc

@@ -3135,6 +3135,9 @@ bool PepperPluginInstanceImpl::FlashSetFullscreen(bool fullscreen,
 }
 
 bool PepperPluginInstanceImpl::IsRectTopmost(const gfx::Rect& rect) {
+  if (is_deleted_)
+    return false;
+
   if (flash_fullscreen_)
     return true;