Stop forcing setPageScaleFactor scrolls to be nonprogrammatic.

It turns out HistoryController calls setPageScaleFactor programmatically
and Mac back-button scroll restore is currently broken due to this.

The only reason we forced these to be user scrolls was in order to avoid
spurious programmatic scroll notifications to Chromium.  We are
currently not using these notifications and plan to delete that whole
system soon (as we've agreed not to ship the ScrollTo() url-bar hiding
behavior if iOS Safari strips it out in iOS7, which they appear on track
to do).

We forgot to cover this quirk with any ProgrammaticScrollTests when we
introduced it, so no test change required.

NOTRY=true
BUG=271136

Review URL: https://chromiumcodereview.appspot.com/24055004

git-svn-id: svn://svn.chromium.org/blink/trunk@157460 bbb929c8-8fbe-4397-9dbb-9b2b20218538