Replace SkConsistentChecksum with SkCityHash (now including CityHash via DEPS)

Alternative to https://codereview.appspot.com/6847087/ ('Change SkConsistentChecksum to use SuperFastHash')
Review URL: https://codereview.appspot.com/6867060

git-svn-id: http://skia.googlecode.com/svn/trunk@6701 2bbb7eff-a529-9590-31e7-b0007b416f81
8 files changed