Make CanvasGradient.addColorStop throw TypeError for non-finite offsets

This aligns with the spec and Gecko.

BUG=367648

Review URL: https://codereview.chromium.org/268443003

git-svn-id: svn://svn.chromium.org/blink/trunk@172988 bbb929c8-8fbe-4397-9dbb-9b2b20218538