re-enable SBNavigationObserverBrowserTest.DownloadViaHTML5FileApi on network_browser_tests

It was accidentally re-disabled in a bad rebase in
https://chromium.googlesource.com/chromium/src/+/3787780199f8ef7bfa5e977aab8180766b10ddad

Bug: 859594
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I2dd6b15dba5639657ddc4deadc4afe6468cd529a
Reviewed-on: https://chromium-review.googlesource.com/1182309
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584817}
1 file changed