Remove false assertion in ApplyBlockElementCommand::formatSelection()

Note: This patch is preparation of fixing issue 294456.

This patch removes false assertion in ApplyBlockElementCommand::formatSelection(), when contents of being indent is modified, e.g. mutation event, |endOfNextParagraph| can hold removed contents.

BUG=294456
TEST=n/a
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/25657004

git-svn-id: svn://svn.chromium.org/blink/trunk@158701 bbb929c8-8fbe-4397-9dbb-9b2b20218538