Revert "Make children indexing implicit by using vector."

This reverts commit dd2eac7b1c3d1f4290df4738343bf7dcf4102f96.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 631781 as the
culprit for failures in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtL2RkMmVhYzdiMWMzZDFmNDI5MGRmNDczODM0M2JmN2RjZjQxMDJmOTYM

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.linux/Cast%20Audio%20Linux/33156

Sample Failed Step: components_unittests

Original change's description:
> Make children indexing implicit by using vector.
> 
> The quad trees we construct are complete by design.
> So we can simply use a vector to index the children of each node.
> 
> This saves space by removing the need to preserve indices (which were ints before).
> However, the trees could now be inflated with additional null leaves (which we didn't save before).
> We believe this results in smaller trees in the aggregate.
> 
> Bug: 929155
> Change-Id: Iaf8a1d61cb239eee32d85aaa518e89363e49a3da
> Reviewed-on: https://chromium-review.googlesource.com/c/1458676
> Reviewed-by: anthonyvd <anthonyvd@chromium.org>
> Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
> Commit-Queue: Alexandre Frechette <frechette@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#631781}

Change-Id: Ia88bf136dbd990b77b76de8c212813359971ed42
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 929155
Reviewed-on: https://chromium-review.googlesource.com/c/1470738
Cr-Commit-Position: refs/heads/master@{#631801}
2 files changed