Use XHR2 `responseType = "json"` when possible

[nicksay]

Chrome and Firefox support setting responseType = "json" on XMLHttpRequest objects: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest#Properties

This allows the browser to parse JSON on a separate thread before returning. Since every request always does a full parse for error-handling (https://github.com/youtube/spfjs/blob/b3c4f07e99f9c5d62f9f3ec59f86c52302edd549/src/client/nav/request.js#L350) using the value provided by responseType = "json" may reduce main thread contention slightly.

This is primarily a concern when loading very large responses.

[nicksay]

Investigated this a little more:

Positive

• JSON parsing done off of the main thread
• Reduces CPU contention; leads to less interference with ongoing animations, etc

Negative

• No accessible errors for malformed responses (i.e. JSON syntax problems)
  • The xhr.response value is null
  • No stack trace, etc
• No support for processing chunked responses on-the-fly (i.e. pseudo-streaming)
  • Using responseType = "json" will prevent sites from effectively using multipart responses.