Assert that loading is complete in SVGImage::hasSingleSecurityOrigin()

This patch adds an assert that the "inner page" in an SVGImage has
loaded and does not have any pending subresource loads.

NOTRY=true
BUG=249037

Review URL: https://chromiumcodereview.appspot.com/22999031

git-svn-id: svn://svn.chromium.org/blink/trunk@157433 bbb929c8-8fbe-4397-9dbb-9b2b20218538