Add 'stream' to XMLHttpRequest response type.

Make XMLHttpRequest return a ReadableStream when the response type is set to
'stream'. This CL also modifies ReadableStream interface and implementation to
make it enable for XMLHttpRequest to use ReadableStream functionalities.

BUG=401396

Review URL: https://codereview.chromium.org/455303002

git-svn-id: svn://svn.chromium.org/blink/trunk@180432 bbb929c8-8fbe-4397-9dbb-9b2b20218538