Remove isDialog() from RenderObject

As RenderDialog.h has been removed, this patch deletes unused isDialog function.
3.65K gain on content_shell release build.

Review URL: https://codereview.chromium.org/27246004

git-svn-id: svn://svn.chromium.org/blink/trunk@159705 bbb929c8-8fbe-4397-9dbb-9b2b20218538