Revert "Revert "Respect existing X-Analytics header, if set""

This reverts commit d5a73c0c10e9286d68db7967a4dc2033c956718a.

As of I569b2ebbc, WebResponse has a 'getHeader' method. The change has been
cherry-picked into both production branches, but I added a method_exists()
check for back-compat.

Change-Id: I213a48c69e5982b654044a17bdb144d31b0c8080
Merge "Revert "Revert "Respect existing X-Analytics header, if set"""