Rename ViewportArguments to ViewportDescription

The original name comes from the fact that the struct was filled with
info from parsing the viewport meta content attribute for key-value
pairs.

Today the struct is used for storing the description of the viewport
and can be filled either from the meta tag of from CSS @viewport
rules.

Review URL: https://codereview.chromium.org/25593006

git-svn-id: svn://svn.chromium.org/blink/trunk@158809 bbb929c8-8fbe-4397-9dbb-9b2b20218538