Refactoring: Change the order of conditions to avoid computing rendererIsEditable()

When we need both Node::renderer() and Node::rendererIsEditable()
conditions to be true to perform some operation, it is more effective
to check for renderer() first, so that if this condition fails we can
avoid unnecessary computation of rendererIsEditable().

All the layout tests have been successfully run with this change.

R=ojan, yosin

Review URL: https://codereview.chromium.org/25490002

git-svn-id: svn://svn.chromium.org/blink/trunk@158856 bbb929c8-8fbe-4397-9dbb-9b2b20218538