[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}