Roll src/third_party/dawn 20c014cee7c3..74cebd68792d (2 commits)

https://dawn.googlesource.com/dawn.git/+log/20c014cee7c3..74cebd68792d

git log 20c014cee7c3..74cebd68792d --date=short --first-parent --format='%ad %ae %s'
2019-11-26 cwallez@chromium.org Allow using swiftshader with the Vulkan backend
2019-11-26 rharrison@chromium.org Remove build rules for spvc support & spirv-cross fuzzers

Created with:
  gclient setdep -r src/third_party/dawn@74cebd68792d

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/dawn-chromium-autoroll
Please CC cwallez@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:dawn-linux-x64-deps-rel;luci.chromium.try:dawn-mac-x64-deps-rel;luci.chromium.try:dawn-win10-x64-deps-rel;luci.chromium.try:dawn-win10-x86-deps-rel
Bug: None
Tbr: cwallez@google.com
Change-Id: Ic26cb4bd6bfc6af8b3e516757c954f3614d30d50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937145Reviewed-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@{#719285}

DEPS

@@ -303,7 +303,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling feed
   # and whatever else without interference from each other.
-  'dawn_revision': '20c014cee7c395a91ab73b58694d1435aae97818',
+  'dawn_revision': '74cebd68792d5788144f50511b3eb35134f9803e',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling feed
   # and whatever else without interference from each other.