Roll external/skia 1798739ab..1749af20a (4 commits)

https://skia.googlesource.com/skia.git/+log/1798739ab..1749af20a

2018-04-13 reed@google.com Revert "add test for degenerate canvas dimension"
2018-04-13 reed@google.com add test for degenerate canvas dimension
2018-04-13 robertphillips@google.com Don't try to readback from the GPU for small blur sigmas
2018-04-13 herb@google.com Make a GM for checking blur bounds.

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Change-Id: I9641bbb48c1b2c2aa5bdbdc0853ebc06cfc59230
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