Roll external/skia a1d1703b7..6382f455f (1 commits)

https://skia.googlesource.com/skia.git/+log/a1d1703b7..6382f455f

2017-08-11 brianosman@google.com Replace log2 with SkNextLog2

Test: Presubmit checks will test this change.
Change-Id: I9b8a70a851a7132f2575566f7a2be14f9f492eb3