Roll external/skia 5fdee443e..37bbfe3ea (2 commits)

https://skia.googlesource.com/skia.git/+log/5fdee443e..37bbfe3ea

2017-09-29 mtklein@chromium.org Make SkASSERT and co. constexpr compatible.
2017-09-29 brianosman@google.com Revert "Revert "Revert "Revert "Make threaded proxy generation MDB-friendly, and defer instantiation""""

Test: Presubmit checks will test this change.
Change-Id: Id0203a79d7890330541a83ca731d440fca43eef5
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