SkBitmapHasher: use 64-bit-truncated MD5 instead of 64-bit CityHash

SkBitmapHasher: use 64-bit-truncated MD5 instead of 64-bit CityHash
BUG=https://code.google.com/p/skia/issues/detail?id=1257

(if we change our mind within the next few days, we can toggle the
BITMAPHASHER_USES_TRUNCATED_MD5 #ifdef ; at some point, we'll remove that
option so we can delete our CityHash implementation entirely)

R=bungeman@google.com

Review URL: https://codereview.chromium.org/14054012

git-svn-id: http://skia.googlecode.com/svn/trunk@8992 2bbb7eff-a529-9590-31e7-b0007b416f81