(no bug) Add comment to clarify the reason of using UniquePtr in nsRubyBaseContainerFrame::Reflow. DONTBUILD, comment-only

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extra : rebase_source : b886f8cb9d0fb104e6a52267213b5afa19de8d08
extra : amend_source : 6c455e8f22ef1ecc58a0f3fe54327d964de85ecb