Remove deprecated/unused layout viewport size.

Removes it from the setter, and removes the getter method entirely.

R=dglazkov

Review URL: https://codereview.chromium.org/298723007

git-svn-id: svn://svn.chromium.org/blink/trunk@174648 bbb929c8-8fbe-4397-9dbb-9b2b20218538