Roll external/skia 578c8ee07..c34d993b6 (3 commits)

https://skia.googlesource.com/skia.git/+log/578c8ee07..c34d993b6

2019-06-18 robertphillips@google.com Revert "Add function to GrDataUtils to handle color conversions."
2019-06-18 robertphillips@google.com Revert "Make SkASSERT not break build in constexpr GrSwizzle method on GCC"
2019-06-18 robertphillips@google.com Revert "Shuffle SkSL sources around so compiler and bytecode can be used w/o GPU"

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Change-Id: Idb328e073cd050d652a1adea8176bdd298dcb1aa
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