Allow SVG images to animate their referenced image

This change makes SVG's <image> element have an animatable
href attribute. Previously, SVGImageElement::imageSourceURL()
returned the non-animating base attribute value but after
this change we return the animating value.

This change required changing the return type of imageSourceURL
from AtomicString& to AtomicString to work around returning a
reference to a local object. This should not have a measurable
impact on performance.

BUG=279445

Review URL: https://chromiumcodereview.appspot.com/23534033

git-svn-id: svn://svn.chromium.org/blink/trunk@157025 bbb929c8-8fbe-4397-9dbb-9b2b20218538