Roll external/skia 78c8f30d6..18e9484ad (4 commits)

https://skia.googlesource.com/skia.git/+log/78c8f30d6..18e9484ad

2017-05-15 robertphillips@google.com Rename finishOpList to finishFlush and call it less frequently
2017-05-15 benjaminwagner@google.com Remove old iPadMini4 jobs.
2017-05-15 mtklein@chromium.org add unaligned_store()
2017-05-15 recipe-roller@chromium.org Roll recipe dependencies (trivial).

Test: Presubmit checks will test this change.
Change-Id: I4685b3b8de4138e785f9e4a0fea5e91981d0e51f