ASSERTION FAILED: factor > 0

If images there are inside templates, we shouldn't parse their sizes, just when the document is active.

BUG=423279

Review URL: https://codereview.chromium.org/653023002

git-svn-id: svn://svn.chromium.org/blink/trunk@184070 bbb929c8-8fbe-4397-9dbb-9b2b20218538