Roll third_party/externals/swiftshader 39a9f1b..920100c (2 commits)

https://swiftshader.googlesource.com/SwiftShader.git/+log/39a9f1bdcd0e..920100cd4246

git log 39a9f1b..920100c --date=short --no-merges --format='%ad %ae %s'
2019-09-02 sugoi@google.com Enable fragmentStoresAndAtomics
2019-09-02 jmadill@chromium.org Fix GN Header checks.

Created with:
  gclient setdep -r third_party/externals/swiftshader@920100cd4246

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/swiftshader-skia-autoroll
Please CC jlavrova@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Debian9-Clang-GCE-GPU-SwiftShader-x86_64-Debug-All-SwiftShader
Bug: None
TBR=jlavrova@google.com
Change-Id: I35b2eb510b29bc49159892429b58f0fc91b85864
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/238782
Reviewed-by: skia-autoroll <skia-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: skia-autoroll <skia-autoroll@skia-public.iam.gserviceaccount.com>

DEPS

@@ -30,7 +30,7 @@ deps = {
   "third_party/externals/spirv-cross"     : "https://chromium.googlesource.com/external/github.com/KhronosGroup/SPIRV-Cross@53ab2144b90abede33be5161aec5dfc94ddc3caf",
   "third_party/externals/spirv-headers"   : "https://skia.googlesource.com/external/github.com/KhronosGroup/SPIRV-Headers.git@29c11140baaf9f7fdaa39a583672c556bf1795a1",
   "third_party/externals/spirv-tools"     : "https://skia.googlesource.com/external/github.com/KhronosGroup/SPIRV-Tools.git@0c4feb643b89d1792b02f7cbef315e9d95633bd7",
-  "third_party/externals/swiftshader"     : "https://swiftshader.googlesource.com/SwiftShader@39a9f1bdcd0eb45cb8813993ae0221b672afaa1c",
+  "third_party/externals/swiftshader"     : "https://swiftshader.googlesource.com/SwiftShader@920100cd4246da21d35f49cbd21bea97616bb1c2",
   #"third_party/externals/v8"              : "https://chromium.googlesource.com/v8/v8.git@5f1ae66d5634e43563b2d25ea652dfb94c31a3b4",
   "third_party/externals/wuffs"           : "https://skia.googlesource.com/external/github.com/google/wuffs.git@6ad7d00a262e862549e4963b4a43d148a8285e50",
   "third_party/externals/zlib"            : "https://chromium.googlesource.com/chromium/src/third_party/zlib@47af7c547f8551bd25424e56354a2ae1e9062859",