Roll third_party/externals/swiftshader 53733a68a49a..15dff3606ebf (3 commits)

https://swiftshader.googlesource.com/SwiftShader.git/+log/53733a68a49a..15dff3606ebf


git log 53733a68a49a..15dff3606ebf --date=short --no-merges --format='%ad %ae %s'
2019-03-08 chrisforbes@google.com Add support for GLSLstd450Cross ExtInst
2019-03-08 sugoi@google.com Descriptor Update Template implementation
2019-03-08 chrisforbes@google.com Fix OpVectorShuffle with mixed vector widths


Created with:
  gclient setdep -r third_party/externals/swiftshader@15dff3606ebf

The AutoRoll server is located here: https://autoroll.skia.org/r/swiftshader-skia-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Debian9-Clang-GCE-GPU-SwiftShader-x86_64-Debug-All-SwiftShader
TBR=reed@google.com

Change-Id: Ia57ded6f89f0ef404cd3d07eab0c036a52be8548
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/199216
Reviewed-by: skia-autoroll <skia-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: skia-autoroll <skia-autoroll@skia-public.iam.gserviceaccount.com>