Remove dead setters in DocumentLoadtiming.

markRedirectStart(), markRedirectEnd() are not used, the corresponding
fields are set in addRedirect().

BUG=376004

Review URL: https://codereview.chromium.org/306073008

git-svn-id: svn://svn.chromium.org/blink/trunk@175383 bbb929c8-8fbe-4397-9dbb-9b2b20218538