Code Cleanup: Live Dev _loadedAgentNames is not accurate.

[redmunds]

This was split off of pull #3203

In LiveDevelopment.js loadAgents() function, agent names are synchronously added to the _loadedAgentNames array after calling agents[name].load(). Seems like they should only be in the list if the call to load() is resolved.

This does not seem to cause any known problem. It just seems to imply that it contains a list of successfully loaded agents and someone may try to use that for something in the future. Possible solutions:

1. If it simply contains the same list as _enabledAgentNames, then the _loadedAgentNames array is not needed
2. If the _loadedAgentNames array is needed, then it should be accurate.
3. The name should be changed to describe what's actually in the array.

[pthiess]

This would be a good thing for the community to look at, therefore I assign it at @dangoor but don't expect him to fix it by himself.

[jasonsanjose]

Also see @iwehrman comment about possible race conditions in loadAgents/unloadAgents here #3203 (comment).

[redmunds]

If any agent doesn't load, then Live Preview fails, so this isn't really an issue. The race condition mentioned by Ian was fixed with _loadAgentsPromise in PR #6889. Closing.