Roll external/skia 2e346a611..784184ab9 (3 commits)

https://skia.googlesource.com/skia.git/+log/2e346a611..784184ab9

2019-04-30 michaelludwig@google.com Prevent outset Ws from going negative
2019-04-30 bungeman@google.com IWYU for gms starting with 'a'.
2019-04-30 egdaniel@google.com Fix initialization of GrVkCaps when using legacy extension flags.

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Change-Id: I5f23e481a56897cde38db0ee3048b151b109ae7f
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