Move cache difference code into remote cache files

BUG=skia:7515

Change-Id: I59e75d460b4ed4d0a737c833520b2335808a4ce4
Reviewed-on: https://skia-review.googlesource.com/115706
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
5 files changed