Make ReactTextComponent properly injectable

ReactTextComponent's implementation is DOM-specific; instead of flattenChildren creating the ReactTextComponent instances, ReactNativeComponent now takes care of having ReactTextComponent injected and creating the element. I also renamed ReactTextComponent to ReactDOMTextComponent and moved it to browser/ui/ where it belongs.

Test Plan: jest, use ballmer-peak example.

Reviewers: @sebmarkbage @jordwalke
I think we can kill this API and special case it in `reactComponentExpect`. You can't get a reference to a text component so this can't be true unless you're using a private API.