Moved remaining legacy viewport tests to unit tests.

In https://codereview.chromium.org/48613005/ most of the
legacy viewport meta tag tests were converted into unit
tests. This CL moves the remaining "trickier" tests.

Note: This relands https://codereview.chromium.org/99223005/

BUG=314001
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/102613002

git-svn-id: svn://svn.chromium.org/blink/trunk@163118 bbb929c8-8fbe-4397-9dbb-9b2b20218538