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These tests extend ExtensionRegistryObserver to wait for extension
load/unload etc. TestExtensionRegistryObserver already provides
this functionality.

Bug: 808136
Test: None
Change-Id: I195f206163f54b7547079b10c48e677a86eaf6db
Reviewed-on: https://chromium-review.googlesource.com/897468Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533967}