Parallel cache.

TBR=reed@google.com

BUG=skia:1330,528560

Committed: https://skia.googlesource.com/skia/+/6f2a486040cb25465990196c229feb47e668e87f

Committed: https://skia.googlesource.com/skia/+/bf2988833e5a36c6b430da6fdd2cfebd0015adec

Review URL: https://codereview.chromium.org/1264103003
9 files changed