[iOS] Add USE(IOSURFACE_CANVAS_BACKING_STORE) to fix build

* platform/graphics/cg/ImageBufferCG.cpp:
(WebCore::ImageBuffer::ImageBuffer): The 'width' and 'height'
variables are only used by code protected by
USE(IOSURFACE_CANVAS_BACKING_STORE).

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@161760 268f45cc-cd09-0410-ab3c-d52691b4dbfc