Allow key events to continue propagation for commands without any event listeners.
Rather than complicating the API with proposals of key re injection, or runtime registration/deregistration (which we might want for other reasons in the future), here's a very simple change that would probably give us the same functionality. If an extension remove all chrome.commands.onCommand listeners, then they will no longer receive the onCommand event and thereafter, the browser should continue propagating the key event and not drop it. I think this is actually the correct behavior, but also allows extension authors to have a little more control over when a command should be processed.
BUG=407163
Review URL: https://codereview.chromium.org/
504183004
Cr-Commit-Position: refs/heads/master@{#292997}