Defer widget updates in ContainerNode::parserRemoveChild.

      ContainerNode::parserRemoveChild is the only consumer of ContainerNode::removeBetween that doesn't defer widget updates during the call. This could potentially lead to problems with scripts running at an inopportune time.

This patch adds a RAII guard that runs deferred widget updates at the end of parserRemoveChild.

BUG=560011

Review URL: https://codereview.chromium.org/1464223002

Cr-Commit-Position: refs/heads/master@{#363154}