Roll src/third_party/swiftshader 6d684b4c7991..18c9ac497117 (3 commits)

https://swiftshader.googlesource.com/SwiftShader.git/+log/6d684b4c7991..18c9ac497117

git log 6d684b4c7991..18c9ac497117 --date=short --no-merges --format='%ad %ae %s'
2019-08-30 capn@google.com Optimize non-solid polygon rasterization
2019-08-30 bclayton@google.com Implement non-solid polygon rasterization
2019-08-30 chrisforbes@google.com Don't use bitcast+float path for integer constant values

Created with:
  gclient setdep -r src/third_party/swiftshader@18c9ac497117

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/swiftshader-chromium-autoroll
Please CC swiftshader-team+autoroll@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=luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_chromium_msan_rel_ng;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
TBR=swiftshader-team+autoroll@google.com

Bug: chromium:b/139872671,chromium:b/139872671
Change-Id: Ief6fbe2c426fac88ec95d1f6db74e0b85464b433
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779381Reviewed-by: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#692253}

DEPS

@@ -178,7 +178,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling SwiftShader
   # and whatever else without interference from each other.
-  'swiftshader_revision': '6d684b4c7991caa8c862c0b30a655f31f8b7b033',
+  'swiftshader_revision': '18c9ac4971176658d2f9f13a043fe93c183556d8',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling PDFium
   # and whatever else without interference from each other.