Make WebPluginContainerImpl::invalidateRect check for NULL renderer

The plugin might attempt to call invalidateRect while it is being
destroyed, in which case its renderer could be NULL.

BUG=432045

Review URL: https://codereview.chromium.org/723733002

git-svn-id: svn://svn.chromium.org/blink/trunk@185308 bbb929c8-8fbe-4397-9dbb-9b2b20218538