Migrator: Fix navigating to top-of-file

There is logic to navigate to the top of a newly-loaded page. It
broke with a visual redesign, in which the title of a page is always
sitting at the top of the screen. The new logic will scroll the page
to the first line of source text.

Change-Id: I685d1fb96c6ca1789f114871acd2bb8148e6ae7e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151032
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

Author: srawlins