FYI, I saw a very similar crash later while debugging a fetch()-related promise in a Worker as well. I didn't look up the crash so I don't know it was the same one, but it manifested in the same way. So this problem probably isn't OffscreenCanvas-specific.

Tentatively bumping to P2 since I think it doesn't require --enable-experimental-canvas-features to trigger.

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/ea7499df6ddbe09bc3c141866d42bebe15605599

commit ea7499df6ddbe09bc3c141866d42bebe15605599
Author: Alexey Kozyatinskiy <kozyatinskiy@chromium.org>
Date: Tue Jan 02 16:43:53 2018

[debugger] do not try to find saved context before DebugEvaluate::Local

Current implementation produces crashes since sometimes saved context
is empty. It looks like we do not need to restore saved context since
we do not set debug context as current, at least all tests are passed.

R=yangguo@chromium.org

Bug:  chromium:797573 ,chromium:792838
Change-Id: I1271640f6a18cbaaecfa1e99ed9ac28e0dbbb1da
Reviewed-on: https://chromium-review.googlesource.com/844979
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50327}
[modify] https://crrev.com/ea7499df6ddbe09bc3c141866d42bebe15605599/src/debug/debug-evaluate.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/2a8e1e4a9470bc3a92c58fde069901497a3f3fed

commit 2a8e1e4a9470bc3a92c58fde069901497a3f3fed
Author: Bill Budge <bbudge@chromium.org>
Date: Tue Jan 02 20:57:32 2018

Revert "[debugger] do not try to find saved context before DebugEvaluate::Local"

This reverts commit ea7499df6ddbe09bc3c141866d42bebe15605599.

Reason for revert: Speculative, but this CL seems correlated with a bunch of timeouts on Linux:

https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20nosnap/builds/19540

Original change's description:
> [debugger] do not try to find saved context before DebugEvaluate::Local
> 
> Current implementation produces crashes since sometimes saved context
> is empty. It looks like we do not need to restore saved context since
> we do not set debug context as current, at least all tests are passed.
> 
> R=​yangguo@chromium.org
> 
> Bug:  chromium:797573 ,chromium:792838
> Change-Id: I1271640f6a18cbaaecfa1e99ed9ac28e0dbbb1da
> Reviewed-on: https://chromium-review.googlesource.com/844979
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50327}

TBR=yangguo@chromium.org,kozyatinskiy@chromium.org

Change-Id: I543201698c96c9762c481c1f6012cc13cb712842
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  chromium:797573 , chromium:792838
Reviewed-on: https://chromium-review.googlesource.com/846205
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50331}
[modify] https://crrev.com/2a8e1e4a9470bc3a92c58fde069901497a3f3fed/src/debug/debug-evaluate.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/781fa72608f8bbcc3ed06bc80e51db0451b5bbfb

commit 781fa72608f8bbcc3ed06bc80e51db0451b5bbfb
Author: Bill Budge <bbudge@chromium.org>
Date: Tue Jan 02 23:38:12 2018

Revert "Revert "[debugger] do not try to find saved context before DebugEvaluate::Local""

This reverts commit 2a8e1e4a9470bc3a92c58fde069901497a3f3fed.

Reason for revert: Time outs were caused by infra issues.

Original change's description:
> Revert "[debugger] do not try to find saved context before DebugEvaluate::Local"
> 
> This reverts commit ea7499df6ddbe09bc3c141866d42bebe15605599.
> 
> Reason for revert: Speculative, but this CL seems correlated with a bunch of timeouts on Linux:
> 
> https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20nosnap/builds/19540
> 
> Original change's description:
> > [debugger] do not try to find saved context before DebugEvaluate::Local
> > 
> > Current implementation produces crashes since sometimes saved context
> > is empty. It looks like we do not need to restore saved context since
> > we do not set debug context as current, at least all tests are passed.
> > 
> > R=​yangguo@chromium.org
> > 
> > Bug:  chromium:797573 ,chromium:792838
> > Change-Id: I1271640f6a18cbaaecfa1e99ed9ac28e0dbbb1da
> > Reviewed-on: https://chromium-review.googlesource.com/844979
> > Reviewed-by: Yang Guo <yangguo@chromium.org>
> > Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#50327}
> 
> TBR=yangguo@chromium.org,kozyatinskiy@chromium.org
> 
> Change-Id: I543201698c96c9762c481c1f6012cc13cb712842
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug:  chromium:797573 , chromium:792838
> Reviewed-on: https://chromium-review.googlesource.com/846205
> Reviewed-by: Bill Budge <bbudge@chromium.org>
> Commit-Queue: Bill Budge <bbudge@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50331}

TBR=bbudge@chromium.org,yangguo@chromium.org,kozyatinskiy@chromium.org

Change-Id: I44e916c796b935c372ce418a3fad44aa25d6f863
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  chromium:797573 , chromium:792838
Reviewed-on: https://chromium-review.googlesource.com/848132
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50332}
[modify] https://crrev.com/781fa72608f8bbcc3ed06bc80e51db0451b5bbfb/src/debug/debug-evaluate.cc