Move canResize to RenderObject

The function is only using RenderObject's and style information
so it isn't specific to RenderLayer. On top of it, the function
was _wrong_ in NULL-checking renderer() (which cannot be NULL).

Review URL: https://chromiumcodereview.appspot.com/24078025

git-svn-id: svn://svn.chromium.org/blink/trunk@158153 bbb929c8-8fbe-4397-9dbb-9b2b20218538