Http Cache: Allow caching of 206s that lack content length.

Servers _should_ include content-length with byte range responses but
relax that requirement because some servers don't do that.

BUG=307535
TEST=net_unittests
R=gavinp@chromium.org

Review URL: https://codereview.chromium.org/38843002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232254 0039d316-1c4b-4281-b951-d872f2087c98

• net/http/http_cache_transaction.cc[diff]
• net/http/http_cache_unittest.cc[diff]
• net/http/partial_data.cc[diff]