Base new scale on minimum-scale after orientation change.

The previous width-ratio based logic was complicated, and behaves badly
in a particular scenario where the contents overflows past the
webmaster's specified minimum scale.  Replace it with some much simpler
logic that just maintains distance from minimum scale.  (In most cases
this behaves the same as before, because minimum scale is usually based
on the ratio of viewport width to contents width.)

NOTRY=true
BUG=395503

Review URL: https://codereview.chromium.org/405263002

git-svn-id: svn://svn.chromium.org/blink/trunk@178608 bbb929c8-8fbe-4397-9dbb-9b2b20218538