SourceCache race condition can cause sources to fail to bubble events up to map

[lbud]

There is a race condition wherein sources that quickly fire events upon construction fail to bubble events to their omniparent (map) because sources are created in SourceCache (source_cache.js#L30), but the SourceCache parent is created before calling setEventedParent(style.js#L376-L382) to add it to the Map -> Style -> SourceCache ownership chain. Generally in our existing source types this isn't a problem, as they usually load data over a network connection and don't race to fire events as quickly (so when they do they already have a map grandparent), but I noticed this problem while working on the canvas source type #3580 which loads its data from the same page.