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Some extension browser tests generate extension objects at runtime.
Update these to generate extensions with manifest_version: 2. Where it
wasn't already used, also update these to use ExtensionBuilder, which
uses manifest v2 by default.

Bug: 816679
Bug: 756488
Change-Id: I7142d085ad9569dccd4e9b62e09c7a68da08e360
Reviewed-on: https://chromium-review.googlesource.com/1008633Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550113}