Do not dispatch events in Document::nodeChildrenWillBeRemoved().

FrameSelection::nodeWillBeRemoved() can dispatch 'beforeload' events
for plugin elements. Avoid it by wrapping Document::
nodeChildrenWillBeRemoved() call with RenderWidget::UpdateSuspendScope.


BUG=312639
TEST=automated
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/45733007

git-svn-id: svn://svn.chromium.org/blink/trunk@160918 bbb929c8-8fbe-4397-9dbb-9b2b20218538