Get BrowserCloseManager browsertests to work with Network Service

The issue is that the tests were using URLRequestSlowDownloadJob,
which doesn't work with the Network Service because it uses
URLRequestInterceptor under the hood. This CL follows the pattern of
https://chromium-review.googlesource.com/c/chromium/src/+/745728 in
switching the tests to use SlowDownloadHttpResponse.

Bug: 776589
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I538c12899dfa5f17b523bd46c83adf16e44c0218
Reviewed-on: https://chromium-review.googlesource.com/873375
Reviewed-by: John Abd-El-Malek <[email protected]>
Reviewed-by: Xing Liu <[email protected]>
Commit-Queue: Colin Blundell <[email protected]>
Cr-Commit-Position: refs/heads/master@{#530214}
2 files changed