parserRemoveChild: Avoid unintended DOM modifications after user script run.

      Surprisingly, ContainerNode::parserRemoveChild may run arbitrary user script during its DOM modification if its target contained iframes.
Before this CL, this could lead to corrupt DOM tree, as the target node could be moved during parserRemoveChild execution.

This CL adds a bail-out if stmt after disconnecting child frame to abort if precondition has changed.

BUG=516377

Review URL: https://codereview.chromium.org/1277793002

git-svn-id: svn://svn.chromium.org/blink/trunk@200098 bbb929c8-8fbe-4397-9dbb-9b2b20218538