Revision 55224

Add the actual data being read to the OnBytesRead callback, take two.

This change was originally committed as
http://src.chromium.org/viewvc/chrome?view=rev&revision=53178
then rolled back as
http://src.chromium.org/viewvc/chrome?view=rev&revision=53416
due to a breakage in FLAKY_UnknownSize download test.
FLAKY_UnknownSize depended on code with a bug that was exposed by this change. That bug has since been fixed and committed as
http://src.chromium.org/viewvc/chrome?view=rev&revision=53876

So this change is ready for another round of review. It has not changed at all since being committed.

Contributed by: bmcquade@google.com

BUG=48192
TEST=Added new unit tests, ran net_unittests.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=54448

Review URL: http://codereview.chromium.org/3010037