Make the Go Back navigation work even when redirection is involved.
Currently, Chrome tries to go back to the page immediately before
the current one. This doesn't work if the current page was visited
by redirection; redirection just occurs again.
With this change, Chrome first tries to find the redirection source
of the current page and then to go back to the page before the source.
Tested: unit_tests, ui_tests, manually.
(unit_tests fails for URLFetcherBadHTTPSTest.BadHTTPSTest,
but I believe it is unrelated to this change.)