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Add an install warning for extensions added with a higher manifest
version than is fully supported. We use a warning (rather than an
error) so the extension still loads, and runs as best it can.

Bug: 950647
Change-Id: I32ae2b21bb8cc83e2ec0effaab6d58324a93a25d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559021Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649235}