Remove isDialog() from RenderObject
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@159705 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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