Move meta content parsing from Document to HTMLMetaElement

Preparation of converting legacy meta "viewport" elements to
CSS styles following the CSS Device Adaptation rules.

Review URL: https://codereview.chromium.org/23606013

git-svn-id: svn://svn.chromium.org/blink/trunk@158737 bbb929c8-8fbe-4397-9dbb-9b2b20218538