Move meta content handling from ViewportDescription to HTMLMetaElement

As a step to convert the viewport meta into CSS @viewport rules,
we move the code related to meta content handling to the HTMLMetaElement
itself.

A few changes was made to the code to make this possible, like removing
document from the arguments and change the method names to better
describe their function.

Review URL: https://codereview.chromium.org/25612004

git-svn-id: svn://svn.chromium.org/blink/trunk@158827 bbb929c8-8fbe-4397-9dbb-9b2b20218538