Roll external/skia 91a1053f2..915893167 (2 commits)

https://skia.googlesource.com/skia.git/+log/91a1053f2..915893167

2017-06-22 jvanverth@google.com Remove static initializer restriction for Vulkan tests
2017-06-22 jvanverth@google.com Blacklist TransferPixelsTest on NVIDIA Shield

Test: Presubmit checks will test this change.
Change-Id: I3e43a0619a8f678e883bbc7df04bfd946e7b78f0