NetworkService: Fix 7 DevToolsDownloadContentTest cases

This CL:
1. Replaced |URLRequestInterceptor| usages with
   |embedded_test_server()| as the former is not supported in
   NetworkService.
2. Introduced |DownloadManager::Observer::OnDownloadDropped()| which is
   called when the download manager intercepted a download navigation
   but didn't create a download item.
3. Removed the following test cases from the filter:
-DevToolsDownloadContentTest.MultiDownload
-DevToolsDownloadContentTest.DefaultDownload
-DevToolsDownloadContentTest.ResetDownloadState
-DevToolsDownloadContentTest.DownloadCancelled
-DevToolsDownloadContentTest.DeniedDownload
-DevToolsDownloadContentTest.DefaultDownloadHeadless
-DevToolsDownloadContentTest.SingleDownload

Bug: 729848
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I9da96982ea8b5d0b4848f438a1f3a5bd8418dd72
Reviewed-on: https://chromium-review.googlesource.com/956560
Reviewed-by: Min Qin <[email protected]>
Reviewed-by: John Abd-El-Malek <[email protected]>
Commit-Queue: Chong Zhang <[email protected]>
Cr-Commit-Position: refs/heads/master@{#542173}
4 files changed