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Reduces complexity from O(n^2) to O(n).  Prior to this commit, we were
performing an O(n) vector::erase for each empty child, say O(n) of
them. Now, we do an amortized constant vector::push_back for each
child to keep, along with O(n) constant-time vector::swap operations.

Bug: 1014388
Change-Id: I4e614463c5ac207dd036876a54e65cca8ce0ed25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863154
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706175}