[trace viewer] Toggle the hotkey help dialog with '?'
[trace viewer] Toggle the hotkey help dialog with '?'

This fixes a bug where pressing '?' multiple times would bring up
multiple help dialogs.

This is https://crrev.com/2142943003/ being submitted to the polymer10
branch.

TBR=aiolos@chromium.org
NOPRESUBMIT=true
NOTRY=true
BUG=catapult:#2466

Review-Url: https://codereview.chromium.org/2143683003

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