Enable expanded GPU rasterization trigger by default

This makes
<meta name="viewport" content="width=device-width, minimum-scale=1.0">
(currently used as the GPU rasterization trigger by a field trial) be
the default trigger for GPU rasterization.

BUG=404866

Review URL: https://codereview.chromium.org/491453002

git-svn-id: svn://svn.chromium.org/blink/trunk@180588 bbb929c8-8fbe-4397-9dbb-9b2b20218538