[NaCl SDK] nacl_io: Fix some httpfs bugs.

* Fix GetStat returning size=0, when there is no "Content-Length" header, and
  cache_stat is off.
* Fix infinite loop when reading uncached past the end of the entity.
* Fix error when doing partial read past the end of the entity.
* Fix Fake URLLoader to handle partial content correctly.
* Add parameterized HTTP tests (on each caching type, cache_stat/cache_content).

BUG=320557
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/81483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236799 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236799