Revert "Use base::AppendToFile instead of local function for appending"

This reverts commit e3a3387021a79452742aee44e58cd4ffd4ed7d79.

Reason for revert: incorrect replacement, as AppendToFile() doesn't
create the file when it doesn't exist - b/352219415

Original change's description:
> Use base::AppendToFile instead of local function for appending
>
> Bug: None
> Change-Id: I88ec1546d17985f516cd60a3bcb67010bfc052a1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5670148
> Reviewed-by: Satoru Takabayashi <satorux@chromium.org>
> Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
> Commit-Queue: Jaewon Jung <jw.jung@navercorp.com>
> Cr-Commit-Position: refs/heads/main@{#1324016}

Bug: b:352219415
Change-Id: I47b7b38ceed08a6e9a433586b151fbe99cbb8104
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5690647
Commit-Queue: Brian Norris <briannorris@chromium.org>
Auto-Submit: Brian Norris <briannorris@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1326186}