Protect documents from deletion when their onload removes them

When an XML document is the src of an iframe, and the onload method
changes the src to something else, the XML document may be garbage
collected before the original load is completed. Bad things result.

In this patch we protect the document in Document::finishedParsing.

R=abarth@chromium.org,eseidel@chromium.org,inferno@chromium.org
BUG=260428

Review URL: https://chromiumcodereview.appspot.com/19962002

git-svn-id: svn://svn.chromium.org/blink/trunk@154680 bbb929c8-8fbe-4397-9dbb-9b2b20218538