Prevent subframe loads during focus-removal in ContainerNode::removeChildren

This fixes a regression from r152990, which re-ordered the calls to
willRemoveChildren() and removeFocusedElementOfSubtree() to fix a different
crash.

R=esprehn
BUG=294202

Review URL: https://chromiumcodereview.appspot.com/24227007

git-svn-id: svn://svn.chromium.org/blink/trunk@158145 bbb929c8-8fbe-4397-9dbb-9b2b20218538