Web Animations CSS: Support animation of webkit-mask-box-[slices|width|outset]

Note: attempting to animate -webkit-mask-box-image in the existing implementation
only animates the source (and not outset, slices, or width). Allowing them to
animate in the web animations implementation of CSS Animations brings this
shorthand in line with border-image.

BUG=257591

Review URL: https://codereview.chromium.org/27280002

git-svn-id: svn://svn.chromium.org/blink/trunk@159731 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Create a new SecurityContext for new documents

When doing document.implementation.createDocument and createHTMLDocument
we used to directly use the SecurityOrigin from document, leading to
mutations through either document would be reflected in other
documents.

Instead of using the same SecurityOrigin we now create a new
SecurityOrigin with the same URL.

BUG=282415
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/27390004

git-svn-id: svn://svn.chromium.org/blink/trunk@159732 bbb929c8-8fbe-4397-9dbb-9b2b20218538