Mark base::Value::Dict::clear and List::clear as reinitializing.

Like their std container counterparts these should be considered as
reinitializing the container after a move, allowing the objects to
be reused.

Bug: none
Change-Id: I8ba8a2daaf9d705b0f2a5f39ceb180f3c241b631
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4099370
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1082612}