Kill WTF::GregorianDateTime class

Kill WTF::GregorianDateTime class and use DateComponents from platform/
instead. There is little sense in having 2 different classes to do the same
thing. This makes the code a bit more consistent and also reduces the binary
size slightly.

R=eseidel@chromium.org, abarth@chromium.org

Review URL: https://codereview.chromium.org/288733002

git-svn-id: svn://svn.chromium.org/blink/trunk@174069 bbb929c8-8fbe-4397-9dbb-9b2b20218538