Use SWITCH/CASE for parsing viewport meta tags

Use the same parsing infrastructure as the CSSParser for parsing
the viewport meta tag. The HTMLMetaElement.cpp is not generated
as a result of this.

Review URL: https://codereview.chromium.org/25810003

git-svn-id: svn://svn.chromium.org/blink/trunk@159025 bbb929c8-8fbe-4397-9dbb-9b2b20218538