[SVG] Missing image loader update on re-insert.

After r167729, SVG images are trying to minimize the number of loader
updates, only firing them on insertion when the URI changes.

But the loader may not even try to fetch the image in some cases
(inactive document for example), leaving the SVG image element in an
invalid state where it doesn't attempt to update the loader again.

The CL expands the SVGImageElement::insertedInto() logic to update the
loader in two cases:

  * dirty URI (current criterion)
  * missing image resource (which signals a failed/incomplete previous
    loader fetch)

BUG=350282
R=pdr@chromium.org

Review URL: https://codereview.chromium.org/191933003

git-svn-id: svn://svn.chromium.org/blink/trunk@168802 bbb929c8-8fbe-4397-9dbb-9b2b20218538