Roll src/third_party/dawn 720988d878a1..e986cb92544a (7 commits)

https://dawn.googlesource.com/dawn.git/+log/720988d878a1..e986cb92544a

git log 720988d878a1..e986cb92544a --date=short --no-merges --format='%ad %ae %s'
2019-10-08 cwallez@chromium.org Vulkan: Add proper error handling for all regular object creation.
2019-10-08 bryan.bernhart@intel.com Fix MSVC build error due to implicit descriptor heap offset cast.
2019-10-08 cwallez@chromium.org Guard macOS or iOS specific code in #if guards
2019-10-08 cwallez@chromium.org Make dawn.h closer to webgpu.h
2019-10-08 cwallez@chromium.org Remove stray push constant code.
2019-10-08 cwallez@chromium.org Allow ResultOrError to downcast from backend to frontend types
2019-10-08 cwallez@chromium.org Fix RenderPassDescriptor to be extensible.

Created with:
  gclient setdep -r src/third_party/dawn@e986cb92544a

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
TBR=cwallez@google.com

Bug: None
Change-Id: I5bedebf75c0f8f3a0a98cc63c1650d53e1e4ab18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847329Reviewed-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@{#703728}

DEPS

@@ -302,7 +302,7 @@ 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': '720988d878a186959e2b799ac8644c82759ef890',
+  'dawn_revision': 'e986cb92544aa2cbf4c5ce30020f024cd25a7344',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling feed
   # and whatever else without interference from each other.