Search+Shift+H keybinding seems to be mapped to 2 events:
1. previous heading if cvox2 is enabled
2. toggles high-contrast mode regardless of cvox2 state

When cvox2 user envokes this command, high-contrast mode is toggled, and previous heading is focused/uttered. I'd argue that cvox2 keybindings should take priority when it's enabled and OS commands should require pass through key.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fa04240e9ca5c26709789a3ebfb5ed522a1d4355

commit fa04240e9ca5c26709789a3ebfb5ed522a1d4355
Author: dtseng <dtseng@chromium.org>
Date: Tue Dec 06 18:36:13 2016

Ensure spoken feedback keyboard hooks prevent re-injection of key up events

Due to the way we handle keyboard events, we need to explicitly handle prevention of both key down and key up events in js.

If we do not, accelerators that happen to trigger via release e.g. the high contrast shortcut, will activate.

BUG= 668771 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2552013003
Cr-Commit-Position: refs/heads/master@{#436653}

[modify] https://crrev.com/fa04240e9ca5c26709789a3ebfb5ed522a1d4355/chrome/browser/resources/chromeos/chromevox/cvox2/background/keyboard_handler.js

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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5406e90c70e3030c25a654c5c056cf74f820bfef

commit 5406e90c70e3030c25a654c5c056cf74f820bfef
Author: David Tseng <dtseng@chromium.org>
Date: Mon Dec 12 18:48:19 2016

Merge to m56: Ensure spoken feedback keyboard hooks prevent re-injection of key up events

Due to the way we handle keyboard events, we need to explicitly handle prevention of both key down and key up events in js.

If we do not, accelerators that happen to trigger via release e.g. the high contrast shortcut, will activate.

BUG= 668771 
TBR=dtseng@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2552013003
Cr-Commit-Position: refs/heads/master@{#436653}
(cherry picked from commit fa04240e9ca5c26709789a3ebfb5ed522a1d4355)

Review-Url: https://codereview.chromium.org/2566253002 .
Cr-Commit-Position: refs/branch-heads/2924@{#462}
Cr-Branched-From: 3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}

[modify] https://crrev.com/5406e90c70e3030c25a654c5c056cf74f820bfef/chrome/browser/resources/chromeos/chromevox/cvox2/background/keyboard_handler.js

verified on 57.0.2957.0