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A bunch of extension unittests generate extension objects at runtime.
Update these to generate extensions with manifest_version: 2. Where
trivial, update these to use ExtensionBuilder (if they weren't already),
which uses manifest v2 by default.

Bug: 816679
Bug: 756488

Change-Id: Iec80a5dd288452e70e5dbdd1902180611f6bbcca
Reviewed-on: https://chromium-review.googlesource.com/1007501
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550180}