Avoid duplicate block-in-inline break tokens.

Regardless of whether the break inside a block-in-inline was to be
resumed in a parallel flow or not, we'd also include it as a break token
in the same flow.

This was usually harmless, since any next line in the same fragmentainer
would typically not be a block-in-inline, since all consecutive blocks
inside inlines are put inside the same block-in-inline wrapper during
box generation.

However, there are ways to end up with adjacent block-in-inline
wrappers: If a block that previously wasn't inflow becomes inflow
(change from float:left to float:none, for instance), a new wrapper is
created around the now-inflow block, even if there's already an adjacent
wrapper there for an adjacent block.

  <span>
    <div id="first" style="float:left;"></div>
    <div id="second"></div>
  </span>

This generates a box tree like this:

  LayoutInline           SPAN
    LayoutNGBlockFlow    (floating) DIV #first
    LayoutNGBlockFlow    (anonymous)
      LayoutNGBlockFlow  DIV #second

If we (after initial layout) change #first to float:none, we end up with
this:

  LayoutInline           SPAN
    LayoutNGBlockFlow    (anonymous)
      LayoutNGBlockFlow  DIV #first
    LayoutNGBlockFlow    (anonymous)
      LayoutNGBlockFlow  DIV #second

The fix is to make sure that we only create one break token for a
fragmented block wrapper. Move the code that examines the outgoing
block-in-inline break token a bit further up, to eliminate the need for
LineInfo::BlockInInlineBreakToken(), and to keep things a bit more
tightly together. Add a DCHECK to LineBreaker::HandleBlockInInline(),
which would have failed without this fix.

Bug: 1495515
Change-Id: I6d019b8c2e6c03046e122fecce76e044f3aeabb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4973967
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1216882}