Extensions: Add curly braces to reduce cognitive load & range-based loop

* Use range-based for loop instead. clang-tidymodernize-loop-convert
* Use emplace_back instead of push_back. clang-tidymodernize-use-emplace

This CL was uploaded by git cl split.

R=hansberry@chromium.org

Bug: chromium:346952289
Change-Id: I9035b94518f2b0562c6978943b100b66c1236c6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5858895
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Solomon Kinard <solomonkinard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1356667}