Currently the test expects that one one read call makes the response
body stream closed, which is not always true. With this change, the test
reads the body stream till the end.

Bug: 603396
Change-Id: I41b6fbb465945702c6374835f2b4f632a1bc2c5c
Reviewed-on: https://chromium-review.googlesource.com/648869
Commit-Queue: Yutaka Hirano <[email protected]>
Reviewed-by: Adam Rice <[email protected]>
Cr-Commit-Position: refs/heads/master@{#499445}