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Add a base::Feature to control Manifest V3 support. If disabled, all
Manifest V3-based extensions will fail to load. This should only be
used as a last resort, and is very much a kill-switch (rather than a
gradual ramp-up tool).

By default, this feature is enabled (allowing manifest v3 extensions).

Bug: 1147344
Change-Id: I3e30ad76fce12bae584a625c81e3759b31d9478e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520333Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826985}