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 is super confusing. At construction time, it's an element and at update time, it's a string.

Can we make it create an element in the update stage too?