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We should not have supported getting a child tree sibling.
Child trees are always considered to be an "only child."

This change removes the ability to traverse to a child
tree using PlatformGet*Sibling, as valid usage of that
method requires a sibling to exist.

PlatformGetChild will now return nullptr if the node has a
child tree and the specified child index is not 0

Updated test.

Bug: 987472
Change-Id: I5b69e169531f3448a317f7b8d698cea5857b7ad7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717912
Commit-Queue: Jacques Newman <janewman@microsoft.com>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682376}