Interestingly, passing the flag to the layout test runner does not trigger any failure:

third_party/WebKit/Tools/Scripts/run-webkit-tests inspector/sources/debugger-step/debugger-step-into-custom-element-callbacks.html --release --additional-driver-flag="--enable-leak-detection --js-flags=--ignition"

Do we have flag-specific test expectations for ignition? I can't find any.

If I hard-code --ignition in flag-definitions.h and use the test runner, we get the failure. If I then add --ignition to --additional-driver-flag, the test passes...

Hmm, that's weird. Re:#1 we do have an Ignition layouttest bot on the V8 FYI waterfall which had its own set of expectations, however we remember that file since we fixed all the failing expectations recently. 

I'm running with hard-coded --ignition and seeing a fairly large number of leaks in LayoutTests with --enable-leak-detection. I don't think --enable-leak-detection is being used in the V8 waterfall. But it certainly causes V8 to get rolled back, if failing.

This is the list of leaks

virtual/service-worker-navigation-preload-disabled/http/tests/serviceworker/chromium/resolve-after-window-close.html
permissionclient/image-permissions.html
virtual/service-worker-navigation-preload/http/tests/serviceworker/chromium/resolve-after-window-close.html
virtual/mojo-loading/http/tests/serviceworker/chromium/resolve-after-window-close.html
virtual/scroll_customization/fast/scroll-behavior/scroll-customization/scrollstate-distribute-to-scroll-chain-descendant.html
virtual/service-worker-navigation-preload-disabled/http/tests/serviceworker/chromium/window-close-during-registration.html
virtual/service-worker-navigation-preload/http/tests/serviceworker/chromium/window-close-during-registration.html
inspector/console/only-one-deprecation-warning.html
fast/loader/data-images-with-images-disabled.html
fast/loader/open-in-srcdoc-unload.html
http/tests/serviceworker/chromium/resolve-after-window-close.html
http/tests/serviceworker/chromium/window-close-during-registration.html
external/wpt/html/webappapis/scripting/events/onerroreventhandler.html
inspector/sources/debugger-step/debugger-step-into-custom-element-callbacks.html
virtual/mojo-loading/http/tests/serviceworker/chromium/window-close-during-registration.html
virtual/scroll_customization/fast/scroll-behavior/scroll-customization/touch-scroll-customization.html

Tests that are expected to fail:
Tests that are expected to fail from src/third_party/WebKit/LayoutTests/LeakExpectations:

virtual/service-worker-navigation-preload-disabled/http/tests/serviceworker/chromium/resolve-after-window-close.html
permissionclient/image-permissions.html
virtual/service-worker-navigation-preload/http/tests/serviceworker/chromium/resolve-after-window-close.html
virtual/mojo-loading/http/tests/serviceworker/chromium/resolve-after-window-close.html
virtual/scroll_customization/fast/scroll-behavior/scroll-customization/scrollstate-distribute-to-scroll-chain-descendant.html
virtual/service-worker-navigation-preload-disabled/http/tests/serviceworker/chromium/window-close-during-registration.html
virtual/service-worker-navigation-preload/http/tests/serviceworker/chromium/window-close-during-registration.html
fast/loader/data-images-with-images-disabled.html
fast/loader/open-in-srcdoc-unload.html
external/wpt/html/webappapis/scripting/events/onerroreventhandler.html
virtual/scroll_customization/fast/scroll-behavior/scroll-customization/touch-scroll-customization.html

Also these tests are expected to fail:

http/tests/serviceworker/chromium/window-close-during-registration.html
http/tests/serviceworker/chromium/resolve-after-window-close.html
virtual/mojo-loading/http/tests/serviceworker/chromium/window-close-during-registration.html

Looking into the following tests that are expected to pass but fail:

inspector/console/only-one-deprecation-warning.html
inspector/sources/debugger-step/debugger-step-into-custom-element-callbacks.html

This is the cl to fix this: https://codereview.chromium.org/2702343003/. I verified locally that it works.