Account for sibling text nodes in CanHaveWhitespaceChildren.

Not doing it results in incorrect behavior when there are multiple adjacent
text nodes, either script-generated ones or because of display: contents.

BUG=746437,657748

Change-Id: If5aa9209d16d5532629a0f0e2f1730d18a193ebf
Reviewed-on: https://chromium-review.googlesource.com/575982
Reviewed-by: Rune Lillesveen <rune@opera.com>
Commit-Queue: Emilio Cobos Álvarez <ecobos@igalia.com>
Cr-Commit-Position: refs/heads/master@{#490971}
6 files changed