[net/dns] Fix how DnsTransaction computes next server index after fallback.

BUG=239657
TEST=net_unittests --gtest_filter=DnsTransactionTest.ServerFallbackAndRotate

Review URL: https://chromiumcodereview.appspot.com/14676012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@199402 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed