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This CL makes the test load an actual extension from test dir instead
of just creating a test extension object that is not loaded in
extension system.

The test flakily fails because the first call to
network::BlockingDnsLookup doesn't resolve dns lookup. This CL is
based on the wild intuition that as we use the extension's url in
NetworkIsolationKey and the extension is not present in extension
system, the test fails. Therefore, loading the extension would
remove the failure cause. Two thousand runs with this fix survives
in bots, where the test wouldn't without this CL.

Bug: 1064832
Change-Id: Id75af472f9457b1205f1caef34b4e6686bbcaeee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2121344
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753533}