Update viewport trigger for GPU rasterization

This updates the viewport trigger for GPU rasterization to
<meta name="viewport" content="width=device-width, minimum-scale=1.0,
  initial-scale=1.0, user-scalable=yes">

BUG=367202

Review URL: https://codereview.chromium.org/268453003

git-svn-id: svn://svn.chromium.org/blink/trunk@173003 bbb929c8-8fbe-4397-9dbb-9b2b20218538