Suppress viewport re-anchoring when resize affects only height

The viewport anchoring logic is responsible for maintaining a consistent scroll
offset and view under logical rotations and non-trivial resizes.  If the resize
only affects viewport height (e.g., appearance of popup keyboard or top
controls), re-anchoring should should not be performed.

BUG=280156

Review URL: https://chromiumcodereview.appspot.com/23411004

git-svn-id: svn://svn.chromium.org/blink/trunk@157010 bbb929c8-8fbe-4397-9dbb-9b2b20218538