Make it possible to generate the ios xcode proj by specifying only skia_os="ios" in GYP_DEFINES.
Make it possible to generate the ios xcode proj by specifying only skia_os="ios" in GYP_DEFINES.

R=caryclark@google.com,epoger@google.com
Review URL: https://codereview.appspot.com/6601051

git-svn-id: e8541e15acce502a64c929015570ad1648e548cd@5796 2bbb7eff-a529-9590-31e7-b0007b416f81

hbwhlklive/color-emoji.skia,Hankuo/color-emoji.skia,Hankuo/color-emoji.skia,Frankie-666/color-emoji.skia,Frankie-666/color-emoji.skia,hbwhlklive/color-emoji.skia,MatChung/color-emoji.skia,MatChung/color-emoji.skia,Hankuo/color-emoji.skia,MatChung/color-emoji.skia,Hankuo/color-emoji.skia,Hankuo/color-emoji.skia,MatChung/color-emoji.skia,Frankie-666/color-emoji.skia,Frankie-666/color-emoji.skia,hbwhlklive/color-emoji.skia,MatChung/color-emoji.skia,Hankuo/color-emoji.skia,Frankie-666/color-emoji.skia,hbwhlklive/color-emoji.skia,hbwhlklive/color-emoji.skia,hbwhlklive/color-emoji.skia,MatChung/color-emoji.skia,Frankie-666/color-emoji.skia