ReadableStreamBytesConsumer should check read results

ReadableStreamBytesConsumer expected that the results from
ReadableStreamReaderDefaultRead should be Promise<Object> because that
is provided from ReadableStream provided by blink, but it's possible to
inject arbitrary values with the promise assimilation.

This CL adds additional checks for such injection.

Bug: 840320
Change-Id: I7b3c6a8bfcf563dd860b133ff0295dd7a5d5fea5
Reviewed-on: https://chromium-review.googlesource.com/1049413
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556751}