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ExtensionBuilder has SetManifestKey(), but "version" is pretty common
(and fairly core to the extension object), so add a convenience method
for setting it.

Add a quick unittest for the same, and update a smattering of places
that previously used SetManifestKey().

Bug: 832958

Change-Id: I55de79360e698f8f9983c24a0e0d5fdb8b3fff9b
Reviewed-on: https://chromium-review.googlesource.com/1242196Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594023}