Create CSSPrimitiveValue from Lengths with zoom handled correctly

This patch adds a new create() method to CSSPrimitiveValue that
constructs primitive values from length values.
Additional code needed to be added to CSSCalcValue to enable calc
lengths to be converted also.

This patch resolves a bug in valueForNinePieceImageQuad() where
Lengths were not having their zoom unapplied when being converted to
CSSPrimitiveValues.

BUG=268927

Review URL: https://chromiumcodereview.appspot.com/22404003

git-svn-id: svn://svn.chromium.org/blink/trunk@156683 bbb929c8-8fbe-4397-9dbb-9b2b20218538