Roll external/skia 9b848d574..a754378ef (4 commits)

https://skia.googlesource.com/skia.git/+log/9b848d574..a754378ef

2017-05-02 egdaniel@google.com Setup support for UNIFORM_TEXEL_BUFFER descriptor sets in Vulkan
2017-05-02 ethannicholas@google.com sksl support for buffer blocks
2017-05-02 jvanverth@google.com Remove EXPERIMENTAL_SHADOWING, Part 1
2017-05-02 scroggo@google.com Revert "Add support for row-by-row jpeg encoding"

Test: Presubmit checks will test this change.
Change-Id: I4b665acc06dd3611bbb249ac2717baadb7555406