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We should reset last text node seen by the whitespace attacher when
entering an option or optgroup element. We normally do that when we see
a layout object, but these element do not have layout objects. We
incorrectly treated them as display:contents during layout tree
rebuild.

This caused a fuzzer DCHECK failure in WhitespaceAttacher where we did
not reset the WhitespaceAttacher for an optgroup which we thought were
a display:contents but not registered as one in the WhitespaceAttacher.

Bug: 855949
Change-Id: I5c341a3f47a4e76216db6f712ecacfc939b5f7fa
Reviewed-on: https://chromium-review.googlesource.com/1113339Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570058}