Kill WTF::DecimalNumber class

Kill WTF::DecimalNumber class and use the Decimal class 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 release binary size
by ~15Kb.

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

Review URL: https://codereview.chromium.org/285643006

git-svn-id: svn://svn.chromium.org/blink/trunk@174260 bbb929c8-8fbe-4397-9dbb-9b2b20218538