Reenable WebContentsImplBrowserTest.DownloadImage_Allow_FileImage with network service.

There have been many fixes in the network service path since this was disabled, and running locally (100x) doesn't show any flakes.

TBR=cmumford@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I80466b2696bd58dc35ae254e47908bad99c1c5ac
Reviewed-on: https://chromium-review.googlesource.com/1028753
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553742}

testing/buildbot/filters/mojo.fyi.network_content_browsertests.filter

@@ -60,9 +60,6 @@
 -CrossSiteDocumentBlockingServiceWorkerTest.NoNetwork
 -CrossSiteDocumentBlockingServiceWorkerTest.NetworkToServiceWorkerResponse
 
-# Flakes https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.linux%2FLinux_Tests__dbg__1_%2F69105%2F%2B%2Frecipes%2Fsteps%2Fnetwork_service_content_browsertests%2F0%2Flogs%2FWebContentsImplBrowserTest.DownloadImage_Allow_FileImage%2F0
--WebContentsImplBrowserTest.DownloadImage_Allow_FileImage
-
 # Flakes on Android.
 # http://crbug.com/827318
 -RenderThreadImplBrowserTest.NonResourceDispatchIPCTasksDontGoThroughScheduler