I have audited the all of the usage, and it is renamed to `GetInProcessSubtreeRootDocument` in bug 1709200 to reduce confusion. So close as FIXED.
Bug 1646516 Comment 8 Edit History
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I have audited the all of the usage, and renamed `GetRootDocument` to `GetInProcessSubtreeRootDocument` in bug 1709200 to reduce confusion. So close as FIXED.