Extensions: Remove custom handling code for special url endpoints.

Remove custom handling code for the embedded test server in ExtensionsAPITest.
Instead use net::test_server::RegisterDefaultHandlers. Also modify a couple of
tests to make them pass.

BUG=None

Change-Id: I500f7abcf79d4a89f6abe6588bacd3894c077f21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497428
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Auto-Submit: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637601}
3 files changed