Inline blocks are always isReplaced()

The ASSERT was first converted to an early return in
http://trac.webkit.org/changeset/2972, and later modified in
http://trac.webkit.org/changeset/4712.

I can't find any instance in which a RenderBlockFlow would be
isInline() and not isReplaced() so seems safe to reinstate the assert.

Review URL: https://codereview.chromium.org/103943012

git-svn-id: svn://svn.chromium.org/blink/trunk@164528 bbb929c8-8fbe-4397-9dbb-9b2b20218538