Remove string.h from SkTypes.h

Change-Id: I2be84f37ea11a386206a96d2fef6e0fc9464e21b
Reviewed-on: https://skia-review.googlesource.com/135264
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
6 files changed