[Android WebView] Treat zero values in viewport arguments specially

For compatibility with the old WebView, zero values in viewport
arguments must not be clamped. Zero values for "width" and "height"
must be interpreted as "device-width" and "device-height", respectively;
and zero "initial-scale" translates into 100%.

BUG=282130

Review URL: https://chromiumcodereview.appspot.com/23691017

git-svn-id: svn://svn.chromium.org/blink/trunk@157140 bbb929c8-8fbe-4397-9dbb-9b2b20218538