TL;DR Moves all plugin registration to AE's extension manager.
The rub is that plugin registration wasn't happening right which cropped up in integration testing. Included is a test that verifies that options are getting properly threaded through to the linter. (This test failed when we had two Extension Managers.)
R=brianwilkerson@google.com, scheglov@google.com
Review URL: https://codereview.chromium.org/1398033004 .
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