I believe this code should work fine in single-process-mash (the conditional just needs to be updated).

Dominic, is there a test that exercises that we could try?

Ryan, could you see about updating the conditional and verifying this works for single-process-mash?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3

commit 50fbbcf24a006c9f0f73e61927a4ddb0e72322b3
Author: Scott Violet <sky@chromium.org>
Date: Mon Oct 29 22:01:09 2018

chromeos: adds InjectEventNoAckNoRewriter()

EventInjector::InjectEventNoAck() goes through EventRewriters (which is what
happens when the event is received from the system). Accessibility code needs
to send events in such a way that they do *not* go through rewriters. This
patch adds such a function.

BUG= 876043 
TEST=covered by tests

Change-Id: I041d433ea5114420f1f98718d9d542947c11dedd
Reviewed-on: https://chromium-review.googlesource.com/c/1302867
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603644}
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/ash/ws/window_service_delegate_impl.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/ash/ws/window_service_delegate_impl.h
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/BUILD.gn
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/event_injector.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/event_injector.h
[add] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/event_injector_unittest.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/event_queue.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/event_queue.h
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/event_queue_unittest.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/host_event_queue.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/host_event_queue.h
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/injected_event_handler.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/public/mojom/event_injector.mojom
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/test_host_event_dispatcher.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/test_ws/test_window_service.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/test_ws/test_window_service.h
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/window_service.h
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/window_service_delegate.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/window_service_delegate.h
[add] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/window_service_test_helper.cc
[add] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/window_service_test_helper.h
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/services/ws/window_service_test_setup.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/ui/aura/window_tree_host_unittest.cc
[modify] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/ui/events/BUILD.gn
[add] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/ui/events/test/test_event_rewriter.cc
[add] https://crrev.com/50fbbcf24a006c9f0f73e61927a4ddb0e72322b3/ui/events/test/test_event_rewriter.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e846f4256e1b625ac982921eb9f335dce68960e0

commit e846f4256e1b625ac982921eb9f335dce68960e0
Author: Scott Violet <sky@chromium.org>
Date: Tue Oct 30 15:42:30 2018

chromeos: makes accessibility inject event circumventing rewriters

This updates the call to event injection so that it does not go through
rewriters.

BUG= 876043 
TEST=covered by tests

Change-Id: Ib4f0190fa26ed66c6004fdde7c66134b4b49bdcd
Reviewed-on: https://chromium-review.googlesource.com/c/1306693
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603913}
[modify] https://crrev.com/e846f4256e1b625ac982921eb9f335dce68960e0/chrome/browser/accessibility/accessibility_extension_api.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e2e627e31875d6c7514459284d7eb9a574cc9c93

commit e2e627e31875d6c7514459284d7eb9a574cc9c93
Author: Evan Stade <estade@chromium.org>
Date: Sat Jan 12 17:01:13 2019

Enable a11y extension api browsertest for mash.

Bug:  876043 
Change-Id: Ie0300e018c8943e6430b5fe8a8f70d53cd1b4b41
Reviewed-on: https://chromium-review.googlesource.com/c/1407648
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622316}
[modify] https://crrev.com/e2e627e31875d6c7514459284d7eb9a574cc9c93/chrome/browser/accessibility/accessibility_extension_api_browsertest.cc
[modify] https://crrev.com/e2e627e31875d6c7514459284d7eb9a574cc9c93/chrome/test/BUILD.gn