Clarify purpose of Length(Undefined) and rename to MaxSizeNone

The Undefined type of Length is only used for representing 'none' in
max-width and max-height. This patch renames Undefined to MaxSizeNone as
the current name is somewhat misleading. The only other usage of it is
in a length which doesn't get accessed.

Review URL: https://codereview.chromium.org/430403002

git-svn-id: svn://svn.chromium.org/blink/trunk@180524 bbb929c8-8fbe-4397-9dbb-9b2b20218538