Revert "Prefer unique_ptr to raw pointer to add child view."

This reverts commit 13e570b6ff4265d67d6be461dd026e5e5de18adb.

Reason for revert: The cl triggers the following DCHECK:
"view.h(407)] Check failed: !view->owned_by_client(). This should only be called if the client is passing ownership of |view| to the parent View."
To trigger the DCHECK navigate to https://rsolomakhin.github.io/pr/gp/ and click on "Buy" button.

Original change's description:
> Prefer unique_ptr to raw pointer to add child view.
> 
> Bug: 648382
> Change-Id: I414ec6f22bdac1d8a219ffbd0185d4911fea3637
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1686974
> Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
> Reviewed-by: anthonyvd <anthonyvd@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#681678}

TBR=anthonyvd@chromium.org,sangwoo108@chromium.org,yongseok.choi@navercorp.com

Change-Id: I011b6b6f24561231b00cf2c3f408c15f5ccb9e6c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 648382
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724462
Reviewed-by: Sahel Sharify <sahel@chromium.org>
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681951}