Properly check for a zero-valued floating-point viewport argument

Zero-valued viewport arguments are given special treatment under certain
circumstances. Fix an issue where argument values less than 1 were incorrectly
cast to zero, resulting in an improperly configured viewport.

BUG=295886,282130

Review URL: https://chromiumcodereview.appspot.com/23754026

git-svn-id: svn://svn.chromium.org/blink/trunk@158197 bbb929c8-8fbe-4397-9dbb-9b2b20218538