Implement enqueue, read close and closed on ReadableStream.

ReadableStream is now specialized to String ChunkType.
It should be generalized in the future.

BUG=393911

Review URL: https://codereview.chromium.org/445453005

git-svn-id: svn://svn.chromium.org/blink/trunk@179598 bbb929c8-8fbe-4397-9dbb-9b2b20218538