Always set the container size for a pAR=none image to the intrinsic size

When updating the viewport for an <svg:image> that has preserveAspectRatio
set to 'none' only update the container size from the intrinsic size, and
never the actual viewport (size) as specified by the 'width' and 'height'
attributes. Previously a change to the actual viewport (when pAR=none)
would cause an update to the container size from the actual viewport size,
since the image/container size was previously set to the intrinsic size.
Also simplify the request for the intrinsic size a bit.

BUG=395139

Review URL: https://codereview.chromium.org/403393002

git-svn-id: svn://svn.chromium.org/blink/trunk@178629 bbb929c8-8fbe-4397-9dbb-9b2b20218538