Fix pinch virtual viewport position after resize.

Associate the viewport anchor with the inner viewport.
Adjust the inner and outer viewport positions after resize
such that they both remain in their allowed range and the
inner viewport origin scales proportionally within the
outer viewport.

As a small cleanup, made the method ScrollView::scrollTo() protected.

BUG=364108

Review URL: https://codereview.chromium.org/556703005

git-svn-id: svn://svn.chromium.org/blink/trunk@182365 bbb929c8-8fbe-4397-9dbb-9b2b20218538