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This change adds a few list utility methods to base::Value. This change
is in preparation for switching the return type of the mutable GetList()
from ListStorage& to base::span<base::Value>, at which point it won't be
possible anymore to remove Values using std::vector's API.

Bug: 646113
Change-Id: Ia10e7f43723722e0a66794466c6ffca0fe221c4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1857118Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705953}