Revert "[perf] Use emplace in LineTruncator::TruncateChild"

This reverts commit fe6b11ef1efadf7a2a47489746aae960fda77193.

Reason for revert: Wash on perf, see results here - https://chromium-review.googlesource.com/c/chromium/src/+/5943622

Original change's description:
> [perf] Use emplace in LineTruncator::TruncateChild
>
> This saves a few assembly instructions, resulting in
> the following minor pinpoint gains:
>
> ArabicLineLayout +1.5% 🙂
> culled-inline-hittest -1.2% 🙂
> floats_show_hide +1.0% 🙂
> multicol_balance-list-items-with-tall-marker +1.3% 🙂
> multicol_lots-of-small-unbreakable-blocks-balanced +1.1% 🙂
> multicol_tall-content-short-columns +1.1% 🙂
>
> No behavior changes are expected.
>
> Change-Id: I33e4a61c2a75e69eda49c5f23194494b6de0fea4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5939972
> Reviewed-by: Koji Ishii <kojii@chromium.org>
> Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
> Cr-Commit-Position: refs/heads/main@{#1370653}

Change-Id: I1ce6c8de2cd535717731c86f478e0e5fa52d7a8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5952446
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1372436}