Editing: Null-check in FrameSelection::updateAppearance

This uncheck causes crash when selecting inside HTMLTextFormElement and either start 
or end node element doesn't have a renderer.

BUG=404560

Review URL: https://codereview.chromium.org/610323003

git-svn-id: svn://svn.chromium.org/blink/trunk@183784 bbb929c8-8fbe-4397-9dbb-9b2b20218538