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Consider this code:
  <style>i { white-space: pre-line }</style>
  X<i> </i><i>&#10;</i>

The first time that NGInlineNode::CollectInlines runs, the text data
will be "X", then "X ", and then AppendForcedBreakCollapseWhitespace
will remove the trailing whitespace before inserting the forced break,
so we get "X\n".

But if some text is appended dynamically, CollectInlines will run again,
and this time it will try to reuse existing items in AppendTextReusing.
Before this patch, this would produce "X \n", which would trigger a
DCHECK failure in ComputeOffsetMapping because it's a different string.

Therefore, this patch prevents AppendTextReusing from reusing existing
items if the last item to collapse with ends with collapsible spaces and
the first item to reuse has 'white-space:pre' and begins with a newline.

Bug: 1136688

TEST=external/wpt/css/css-text/white-space/pre-line-with-space-and-newline.html

Change-Id: I0ff71b125b66256ec3b5c263605c3c116f9ac55f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466279Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Cr-Commit-Position: refs/heads/master@{#816641}