Replace static_cast with safe casting methods

Continue Julien's work on reducing static_cast for 3 most popular CSS
casts (CSSPrimitiveValue, CSSValueList and CSSImageValue).

BUG=269140

Review URL: https://chromiumcodereview.appspot.com/22438008

git-svn-id: svn://svn.chromium.org/blink/trunk@155689 bbb929c8-8fbe-4397-9dbb-9b2b20218538