Change zoom handling in hittest-relative-to-viewport test

Change the hittest-relative-to-viewport.html test to scale the hit test
coordinates when zooming instead of having two sets of hard coded
values.

This is more reliable than comparing the resulting offsets as a
rounding error will at most be off by one now.

Review URL: https://chromiumcodereview.appspot.com/23721004

git-svn-id: svn://svn.chromium.org/blink/trunk@156886 bbb929c8-8fbe-4397-9dbb-9b2b20218538