Implement MoveMigrator.

This patch is part of a refactor to make `BrowserDataMigratorImpl` only
responsible for high level coordination of the migration such as
determining if migration should happen and initiating the migration. The
actual migration will be handled by two separate classes, one for copy
migration and one for move migration.

This patch creates a new class `MoveMigrator` which implements the move
migration logic.

Test: unit_tests
Bug: 1261730
Change-Id: I842eb53ca8e0f0f2cbcb43da110ab626c51f410e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3428525
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#969316}