Factor out common BorderImageLength code in CSSToStyleMap

CSSToStyleMap::mapNinePieceImageQuad had a lot of repetitive code for
BorderImageLength. Factor it out to a static function.

Afterwards, the default constructor for BorderImageLengthBox gets used
less so make the code more explicit by removing the default
constructor entirely. Instead add a constructor that initializes the
members to a specific value.

Since this revealed one error in a code comment (the initial value for
-webkit-mask-box-image-width is 'auto' not zero) fix that as well.

BUG=322014

Review URL: https://codereview.chromium.org/81123002

git-svn-id: svn://svn.chromium.org/blink/trunk@162646 bbb929c8-8fbe-4397-9dbb-9b2b20218538