Fix render tree construction when inlines are inserted in reverse after a block in a continuation

When inserting inlines after a block inside of an inline continuation, if we insert inline
content in reverse DOM order, we can (depending on when we attach) end up appending the renderers
in the wrong location, leading to visual aberrations. Since Blink r158839, we always attach in
reverse DOM order, which makes this bug easier to run into.

The fix is in RenderInline, where we should always attach behind our beforeChild if we're
inserting an inline and beforeChild is an inline, regardless of other continuation state.

Big thanks to Elliott for the test case!

BUG=318805

Review URL: https://codereview.chromium.org/68013006

git-svn-id: svn://svn.chromium.org/blink/trunk@162160 bbb929c8-8fbe-4397-9dbb-9b2b20218538