Http cache: Avoid resuming (and keeping) truncated entries
if the server doesn't provide a strong validator.

We require:
- A strong etag or a strong last modified date.
- The total Content length.
- Lack of an explicit rejection of ranges (Accept-ranges: none) 

This aligns better with the conditions used by Firefox.

BUG=30220, b/2329250
TEST=unittests

Review URL: http://codereview.chromium.org/517043

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@35761 0039d316-1c4b-4281-b951-d872f2087c98

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