Expose replacesCurrentHistoryItem() on WebDataSource.

Currently, there is a WebDataSource::isClientRedirect(), but it actually returns
whether or not the back forward list should be locked for the current load.
Start the process of clarify the two.

BUG=261374

Review URL: https://chromiumcodereview.appspot.com/19675005

git-svn-id: svn://svn.chromium.org/blink/trunk@154503 bbb929c8-8fbe-4397-9dbb-9b2b20218538