Enable extensions with parental approval for Linux/Mac/Windows.

Compiles in the code for all platforms, but flag-guards it for new
platforms. Chrome OS is soft-guarded by constexpr if-statement.

Extension flow works for new platforms in-par with ChromeOS,
specifically, prompts the password during first install, but allows
enabling and disabling already present extension.

Bug: b:266051970
Change-Id: Ic74ff4f8ad3d67da455b260d8ac454c7f78fcde3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4527675
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: James Lee <ljjlee@google.com>
Commit-Queue: Tomek Jurkiewicz <tju@google.com>
Cr-Commit-Position: refs/heads/main@{#1151737}