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
This can actually be an invariant since it doesn't add any runtime cost.