Return raw CSSValue pointer instead of cast to CSSImageValue

Avoid an assertion triggering detected when CSSValue pointer is either
void or not cast to CSSImageValue. As far as I've seen, this data()
function is used to compare objects in RenderStyle, so it makes sense to
return a raw pointer.

The assertion avoids successful run of the following layout tests:
fast/hidpi/image-set-out-of-order.html
fast/hidpi/image-set-without-specified-width.html
virtual/gpu/fast/hidpi/image-set-out-of-order.html
virtual/gpu/fast/hidpi/image-set-without-specified-width.html

BUG=269627

Review URL: https://chromiumcodereview.appspot.com/22631003

git-svn-id: svn://svn.chromium.org/blink/trunk@155874 bbb929c8-8fbe-4397-9dbb-9b2b20218538