Move TrailingObjets class into its own file

As a part of my RenderBlockLineLayout (BreakingContextInlineHeaders) refactoring
campaign, in this patch I'm separating TrailingObjects class into its own file.

BUG=335347

Review URL: https://codereview.chromium.org/134313009

git-svn-id: svn://svn.chromium.org/blink/trunk@165346 bbb929c8-8fbe-4397-9dbb-9b2b20218538